Al-Adab Al-Mufrad, Book 45, Hadith 4 (In-Book)

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'"

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