Al-Adab Al-Mufrad 1125
Anas ibn Sirin said, "I wrote for Ibn 'Umar and he said, 'Write:
"In the Name of Allah, the All-Merciful, Most Merciful. Following on from
that: To so-and-so."'"
وَعَنِ ابْنِ عَوْنٍ، عَنْ أَنَسِ بْنِ سِيرِينَ قَالَ‏:‏ كَتَبْتُ لِابْنِ عُمَرَ، فَقَالَ‏:‏ اكْتُبْ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ، أَمَّا بَعْدُ‏:‏ إِلَى فُلانٍ‏.‏