(45) Letters
كتاب الرَّسَائِلِ‏
(528) Chapter: The one who is put first in a letter
(528) بَابُ‏:‏ بِمَنْ يَبْدَأُ فِي الْكِتَابِ‏؟‏
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.‏
Grade :  Sahih (Al-Albani) صـحـيـح  (الألباني)حكم :
Reference : Al-Adab Al-Mufrad 1125
In-book reference : Book 45, Hadith 9
English translation : Book 45, Hadith 1125