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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.'"
Source: sunnah.com
Other translations
Arabic
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.
sunnah.com