Musannaf Ibn Abi Shayba 17:548

'Umar ibn Sa'd narrated to us, from Sufyan, from Ja'far ibn Burqan, from 'Umar ibn 'Abd al-'Aziz: That he wrote in a letter: "As for what follows" (Amma ba'd). Then he said: "It was in the letters of the Prophet, may Allah bless him and grant him peace: 'As for what follows' (Amma ba'd)."