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Zayd ibn Aslam said, "My father sent me to Ibn 'Umar and I saw him write, 'In the Name of Allah, the All-Merciful, Most Merciful. Following on from that'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا قَبِيصَةُ، قَالَ: حَدَّثَنَا سُفْيَانُ، عَنْ زَيْدِ بْنِ أَسْلَمَ قَالَ: أَرْسَلَنِي أَبِي إِلَى ابْنِ عُمَرَ، فَرَأَيْتُهُ يَكْتُبُ: بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ، أَمَّا بَعْدُ.
sunnah.com