It is narrated from Sayyiduna Zaid bin Thabit (may Allah be pleased with him) that he wrote this letter: «بسم الله الرحمن الرحیم». From Zaid bin Thabit to the servant of Allah, the Commander of the Faithful, Muawiyah. Peace and mercy of Allah be upon the Commander of the Faithful. I praise Allah, besides whom there is no true deity. To proceed.
Hadith Referenceالادب المفرد / كتاب الرسائل / 1122
Hadith Gradingالألبانی:حسن
Hadith Takhrij«حسن : أخرجه مالك فى الموطأ كما فى رواية محمد بن الحسن عنه : 901 و الطبراني فى الكبير : 134/5 و البيهقي فى الكبرىٰ : 247/6»
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