It is Mustahab (recommended) to recite Basmalah (Bismillâhirrahmânirrahim) at the beginning of any good deed.
Rasûlullâh (sallallâhu alayhi wa sallam) said: “(The very first thing Allâh Ta‘âlâ created was the Pen.) The first thing the Pen wrote was “Bismillâhirrahmânirrahim.” And when you write down something, write Basmalah first. It is the key of every scripture sent down (to the prophets alayhimus salâm). When Jibril (alayhis salâm) brought it to me, he repeated it three times and said: “This is for you and your Ummah (community). Enjoin your Ummah not to neglect it before beginning any act. I, myself, have not neglected (to read) it even for a moment since I brought it down to your father, Âdam Alayhis Salâm, nor have the other angels.”
The Prophet of Allâh (sallallâhu alayhi wa sallam) said to Abû Hurayrah radiyallâhu anhu: “O, Abû Hurayrah! Say “Bismillâh” before making Wudû, for, if you do so, the Hafaza (guardian) angels continue writing Thawâb (reward) for you until you complete your Wudû.”
Moreover, Anas bin Mâlik (radiyallâhu anhu) reported, “Rasûlullâh (sallallâhu alayhi wa sallam) said: ‘Reciting Bismillâhirrahmânirrahim before taking off clothes serves as a veil against Jinns (spirits or demons) to have access to see the private parts of human beings.”
Rasûlullâh (sallallâhu alayhi wa sallam) is reported to have said: “On the Day of Judgement, one Rak‘ah (unit) Salâh of the Ummah of Muhammad (sallallâhu alayhi wa sallam) will be superior in rewards to a thousand Rak‘ah Salâh of the Banû Isrâîl. When they ask Allâh Ta‘âlâ about its wisdom, Allâh Ta‘âlâ will say: “Because their Salâh (ritual prayers) contained Bismillâhirrahmânirrahim.”
Abû Muslim Hawlani (rahmatullâhi alayhi), from the Tabi‘în, had a maid-servant who attempted to kill him a few times by putting poison in his drinks. However, the poison did not harm him in the least. After some time, the servant confessed his evil deed, as he was astounded, and asked his master why the poison did not work on him. Hawlani rahmatullâhi alayhi said: “I say ‘Bismillâhirrahmânirrahim’ sincerely before eating or drinking anything. That’s why nothing from the things I eat and drink harms me.”
Rasûlullah (sallallâhu alayhi wa sallam) said: “Whenever Jibrâîl (alayhis salâm) came to me to bring revelation, he would first read unto me ‘Bismillâhirrahmânirrahîm’” (Sunan Dâraqutnî)