Hadith 45:10
Sahih (Al-Albani)
Anas ibn Sirin said, "A man wrote in the presence of Ibn 'Umar, 'In the Name of Allah, the All-Merciful, Most Merciful, to so-and-so.' Ibn 'Umar forbade him and said, 'Say:
'In the Name of Allah. It is for Him.'"
وَعَنِ ابْنِ عَوْنٍ، عَنْ أَنَسِ بْنِ سِيرِينَ قَالَ: كَتَبَ رَجُلٌ بَيْنَ يَدَيِ ابْنِ عُمَرَ: بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ، لِفُلاَنٍ، فَنَهَاهُ ابْنُ عُمَرَ وَقَالَ: قُلْ: بِسْمِ اللهِ، هُوَ لَهُ
Original Reference: Al-Adab Al-Mufrad 1126