Al-Adab Al-Mufrad, Hadith 1126 ⓘ
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.
also referenced under: Book 45, Hadith 10 (In-Book) & Book 45, Hadith 1126 (English)