Hadith 45:9
Sahih (Al-Albani)
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
Original Reference: Al-Adab Al-Mufrad 1125