Hadith 17:550
Abu Usama narrated to us, from Hisham, who said: I read in one of the letters of the Prophet, may Allah bless him and grant him peace: "Whenever a matter concluded, he would say: 'Amma ba'd' (As for what follows)."
حدثنا أبو أسامة عن هشام قال قرأت في رسائل من رسائل النبي صلى الله عليه وسلم كلما انقضى امر قال أما بعد