Musannaf Ibn Abi Shayba 3:606

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Abu Wa’il, from Abdullah, who said: We used to pray behind the Prophet, may God bless him and grant him peace, and we would say: "Peace be upon God before His servants, peace be upon Gabriel, peace be upon Michael, peace be upon such-and-such and such-and-such." When the Messenger of God, may God bless him and grant him peace, finished, he said: "Indeed, God is Peace (As-Salam). So when one of you sits in his prayer, let him say: 'The greetings are for God, and the prayers are for God, and the good things are for God. Peace be upon you, O Prophet, and the mercy of God and His blessings. Peace be upon us and upon the righteous servants of God. I bear witness that there is no god but God, and I bear witness that Muhammad is His servant and His messenger.' Then he may choose [whatever supplication he wishes]."