حَدَّثَنَا رَوْحُ بْنُ عَبْدِ الْمُؤْمِنِ، قَالَ: حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامِ بْنِ عُرْوَةَ قَالَ: رَأَيْتُ رَسَائِلَ مِنْ رَسَائِلِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، كُلَّمَا انْقَضَتْ قِصَّةٌ قَالَ: ”أَمَّا بَعْدُ.“
It is narrated from Hisham bin Urwah (may Allah have mercy on him) that I have seen several letters of the Prophet (peace be upon him) in which, when a matter was concluded, "Amma Ba'd" was written there.