When you say "Bismillah" before anything you do, you're invoking the Name of Allah on that activity; not only will that activity be blessed but shaytaan cannot take part in it!

So always say "Bismillah" before anything you do! SubhanaAllah, it's easy for us to forget to say "Bismillah." Sometimes we're so used to saying it that we cannot remember whether we said it or not!

Try to be conscious of saying "Bismillah" and understand what you're saying before your action is performed.

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"Bismillah-ir-Rahman-ir-Raheem" is the first verse of the first chapter (Surah Al-Fatihah) of the Qur'an. The fact that the word of Allah, the Qur'an, starts with this first verse, alone signifies the importance of this verse and further emphasizes the need to start all our tasks in the name of Allah.

Every good action or deed should be started with reciting Bismillah.

When Prophet Sulaiman (A.S) wrote the letter to the Queen, he started it with Bismillah-ir-Rahman-ir-Raheem, as mentioned in the Qur'an itself.

29. (The Queen of Sheba) said (when she received the letter): O chieftains! Lo! there hath been thrown unto me a noble letter.
30. Lo! it is from Solomon, and lo! it is: In the name of Allah, the Beneficent, the Merciful;
[Al-Quran, Surah Al-Naml, chapter 27, Verses 29-30]

Some places where you should never forget saying Bismillah as mentioned in the Ahadeeth.

1) Saying Bismillah Is Recommended When Entering The House:

Jabir (May Allah be pleased with him) reported:
I heard Messenger of Allah (ﷺ) saying, "If a person mentions the Name of Allah upon entering his house or eating, Satan says, addressing his followers: 'You will find no where to spend the night and no dinner.' But if he enters without mentioning the Name of Allah, Satan says (to his followers); 'You have found (a place) to spend the night in, and if he does not mention the Name of Allah at the time of eating, Satan says: 'You have found (a place) to spend the night in as well as food."'
[Muslim].
Sunnah.com reference : Book 3, Hadith 3
Arabic/English book reference :Book 3, Hadith 730

2) Saying Bismillah Is Recommended When Leaving The House:

The Messenger Of Allah (S.A.W.S) said,
"If anyone of you when leaving the house says In the name of Allah, I trust in Allah, there is no power and might except from Allah. Your needs shall be fulfilled, you shall be saved from difficulties and hardships. Shaitaan hearing these words leaves him."
[Al-Trimidhi 2/493]

3) Saying Bismillah Is Recommended Before Eating:

'Umar bin Abu Salamah (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ), said to me, "Mention Allah's Name (i.e., say Bismillah before starting eating), eat with your right hand, and eat from what is near you."
[Al-Bukhari and Muslim].
Sunnah.com reference     : Book 3, Hadith 1
Arabic/English book reference :Book 3, Hadith 728

'Aishah (May Allah be pleased with her) reported:
Messenger of Allah (ﷺ) said, "When any of you wants to eat, he should mention the Name of Allah in the begining, (i.e., say Bismillah). If he forgets to do it in the beginning, he should say Bismillah awwalahu wa akhirahu (I begin with the Name of Allah at the beginning and at the end)."
[At-Tirmidhi and Abu Dawud].
Sunnah.com reference : Book 3, Hadith 2
Arabic/English book reference :Book 3, Hadith 729

Hudhaifah (May Allah be pleased with him) reported:
When we attended a meal with the Messenger of Allah (ﷺ), we would not stretch forth our hands towards the food until he (ﷺ) would start eating first. Once, we were with him when a little girl rushed in as if someone was impelling her. She was about to lay her hand on the food when the Messenger of Allah (ﷺ) caught her hand. Then a bedouin came in rushing as if someone were pushing him. He (ﷺ) caught his hand also and said, "Satan considers that food lawful for himself on which the Name of Allah is not mentioned. He (Satan) brought this girl to make the food lawful through her but I caught her hand. Then he brought the bedouin to make it lawful through him but I caught his hand too. By Him in Whose Hand my soul is, now Satan's hand is in my grasp along with their hands." Then he mentioned the Name of Allah and began to eat.
[Abu Dawud and An-Nasa'i].
Sunnah.com reference : Book 3, Hadith 4
Arabic/English book reference :Book 3, Hadith 731

Umaiyyah bin Makhshi (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ) was sitting while a man was eating food. That man did not mention the Name of Allah (before commencing to eat) till only a morsel of food was left. When he raised it to his mouth, he said: "Bismillah awwalahu wa akhirahu (With the Name of Allah, in the beginning and in the end)." Messenger of Allah (ﷺ) smiled at this and said, "Satan had been eating with him but when he mentioned the Name of Allah, Satan vomited all that was in his stomach."
[Abu Dawud and An-Nasa'i].
Sunnah.com reference : Book 3, Hadith 5
Arabic/English book reference :Book 3, Hadith 732

'Aishah (May Allah be pleased with her) reported:
Messenger of Allah (ﷺ) was eating with his six Companions when a desert Arab came and ate up the food in two mouthfuls. Messenger of Allah (ﷺ) said, "Had he mentioned the Name of Allah, it would have sufficed for all of you."
[At-Tirmidhi].
Sunnah.com reference : Book 3, Hadith 6
Arabic/English book reference :Book 3, Hadith 733

'Umar bin Abu Salamah (May Allah be pleased with them) reported:
I was a boy under the care of Messenger of Allah (ﷺ), and as my hand used to wander around in the dish, he (ﷺ) said to me once, "Mention Allah's Name (i.e., say Bismillah), eat with your right hand, and eat from what is in front of you."
[Al-Bukhari and Muslim].
Sunnah.com reference : Book 3, Hadith 13
Arabic/English book reference :Book 3, Hadith 740

Wahshi bin Harb (May Allah be pleased with him) reported:
Some of the Companions of Messenger of Allah (ﷺ) said: "We eat but are not satisfied." He (ﷺ) said, "Perhaps you eat separately." The Companions replied in affirmative. He then said: "Eat together and mention the Name of Allah over your food. It will be blessed for you."
[Abu Dawud].
Sunnah.com reference : Book 3, Hadith 16
Arabic/English book reference :Book 3, Hadith 743

Ibn 'Abbas (May Allah be pleased with them) reported:
The Prophet (ﷺ) said, "Blessing descends upon food in its middle, so eat from the sides of the vessel and do not eat from its middle."
[At-Tirmidhi and Abu Dawud].
Sunnah.com reference : Book 3, Hadith 17
Arabic/English book reference :Book 3, Hadith 744

Jabir (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ) commanded the licking of fingers and the gleaning of the dish, saying, "You do not know in which portion the blessing lies."
[Muslim].
Sunnah.com reference : Book 3, Hadith 23
Arabic/English book reference :Book 3, Hadith 750

Ibn 'Abbas (May Allah be pleased with them) reported:
Messenger of Allah (ﷺ) said, "When one of you finishes eating, he should not wipe his fingers until he has licked them himself or has given them to someone else to lick for him."
[Al-Bukhari and Muslim].
Sunnah.com reference : Book 3, Hadith 21
Arabic/English book reference :Book 3, Hadith 748

Ka'b bin Malik (May Allah be pleased with him) reported:
I saw Messenger of Allah (ﷺ) eating with three fingers (i.e., the thumb, the index finger and the middle finger) and licking them after having finished the food.
[Muslim].
Sunnah.com reference : Book 3, Hadith 22
Arabic/English book reference :Book 3, Hadith 749

Jabir (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ) said, "When a morsel of any of you falls, he should pick it up and remove any of the dirt on it and then eat it, and should not leave it for Satan nor should wipe his hand with towel until he has licked his fingers, for he does not know in what portion of the food the blessing lies."
[Muslim].
Sunnah.com reference : Book 3, Hadith 24
Arabic/English book reference :Book 3, Hadith 751

Anas (May Allah be pleased with him) reported:
Whenever Messenger of Allah (ﷺ) ate food, he would lick his three fingers and say, "If anyone of you drops a morsel of food, he should remove any dirt that may have stuck on it and then eat it, and should not leave it for Satan." He (ﷺ) also commanded us that we should glean the pot, saying, "You do not know in which part of your food the blessings lies."
[Muslim].
Sunnah.com reference : Book 3, Hadith 26
Arabic/English book reference :Book 3, Hadith 753

4) Saying Bismillah Is Recommended When Drinking Water:

Ibn 'Abbas (May Allah be pleased with them) reported:
Messenger of Allah (ﷺ) said, "Do not drink in one gulp like a camel, but in two or three (gulps). Mention the Name of Allah (i.e., say Bismillah) when you start drinking and praise Him (i.e., say Al-hamdu lillah) after you have finished (drinking)."
[At- Tirmidhi].
Sunnah.com reference : Book 3, Hadith 31
Arabic/English book reference :Book 3, Hadith 758

5) The Virtue Of Saying Bismillah:

Uthman b. Abu al-'As Al-Thaqafi reported that he made a complaint of pain to Allah's Messenger (ﷺ) that he felt in his body at the time he had become Muslim. Thereupon Allah's Messenger (ﷺ) said:
Place your hand at the place where you feel pain in your body and say Bismillah (in the name of Allah) three times and seven times A'udhu billahi wa qudratihi min sharri ma ajidu wa uhadhiru (I seek refuge with Allah and with His Power from the evil that I find and that I fear).
Reference    : Sahih Muslim 2202
In-book reference    : Book 39, Hadith 91
USC-MSA web (English) reference    : Book 26, Hadith 5462

6) Saying Bismillah Is Recommended At The Beginning Of Ablution:

The Prophet (S.A.W.S) said, "Make Wudu [Ablution] in the name of Allah."
[An-Nasai 1/11 and Ibn Khuzaimah 1/84]

7) Saying Bismillah Is Recommended Before Having Sexual Intercourse with one's wife:

The Prophet (S.A.W.S) said,
"If anyone of you, when having sexual intercourse with his wife, says: In the name of Allah. O Allah! Protect us from Satan and also protect what you grant us [i.e. Coming offspring] from Satan and if it is destined that they should have a child, then Satan will never be able to harm him."
[Sahih Bukhari 7/94 and Sahih Muslim 2/1058]

8) Say Bismillah At The Time When Slaughtering An Animal:

Anas (R.A) reported that the Messenger (S.A.W.S) of Allah sacrificed two horned rams of white color with black markings over them. He also stated: I saw him sacrificing them with his own hand and saw him placing his foot on their sides, and recited in the name of Allah and Glorified Him [Bismillah Allaahu Akbar]
[Bukhari 7/133, Muslim 2/181, Abu Dawud 3/230, An-Nasai 7/231, Al-Trimidhi 5/76, Ibn Majah 2120]

9) Saying Bismillah Is Recommended When Fighting For The Cause Of Allah:

The Messenger of Allah (S.A.W.S) said, "Fight in the name of Allah and in the way of Allah"
[Sahih Muslim 2/69]

10) Saying Bismillah Is Recommended When Night Falls:

Allah's Apostle (S.A.W.S) said, "When night falls stop your children from going out, for the devils spread out at that time. But when an hour of the night has passed, release them and close the doors and mention Allah's Name, for Satan does not open a closed door.
Tie the mouth of your water-skin and mention Allah's Name; cover your containers and utensils and mention Allah's Name. Cover them even by placing something across it, and extinguish your lamps. "
[Bukhari 7/527 and Abu Dawud 3722]

11) Saying Bismillah Is Recommended When Placing The Body Of The Deceased In The Grave:

The Prophet of Allah (S.A.W.S) said, "When you are lowering the deceased in the grave you should say In the name of Allah and in accordance to the Sunnah of the Messenger of Allah."
[Ahmad 8/58 and Al-Hakim 1/366]

May Allah (S.W.T) make us amongst those who always remember to mention Allah's name in any work.