It is narrated from Abu Mas’ud Jariri (may Allah have mercy on him) that a man asked Hasan al-Basri (may Allah have mercy on him) about reciting «بسم الله», so he said: It should also be written at the beginning of letters.
Hadith Referenceالادب المفرد / كتاب الرسائل / 1123
Hadith Gradingالألبانی:صحيح الإسناد عن الحسن وهو البصري
Hadith Takhrij«صحيح الإسناد عن الحسن وهو البصري»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: From this, it is understood that every good deed should be initiated with "Bismillah ar-Rahman ar-Raheem." This is a means of goodness and blessing.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 1123