Al-Adab Al-Mufrad, Hadith 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.
also referenced under: Book 45, Hadith 9 (In-Book) & Book 45, Hadith 1125 (English)