The one who is put first in a letter Al-Adab Al-Mufrad 1125
Arabic hadith text
وَعَنِ ابْنِ عَوْنٍ، عَنْ أَنَسِ بْنِ سِيرِينَ قَالَ: كَتَبْتُ لِابْنِ عُمَرَ، فَقَالَ: اكْتُبْ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ، أَمَّا بَعْدُ: إِلَى فُلانٍ.
Translation
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 fromthat: To so-and-so."'"