Musnad Abu Ya'la al-Mawsili 152:67

Abu Khaythama narrated to us, Jarir (meaning Ibn Abd al-Hamid) narrated to us, from Mansur, from Abu Wa'il, from Abdullah, who said: We used to say during the prayer behind the Messenger of Allah (may Allah bless him and grant him peace), "Peace be upon so-and-so." The Messenger of Allah (may Allah bless him and grant him peace) said to us one day: "Indeed, Allah is As-Salam (the Peace). So when one of you sits in the prayer, let him say: 'The greetings are for Allah, and the prayers, and the good things. Peace be upon you, O Prophet, and the mercy of Allah. Peace be upon us and upon the righteous servants of Allah.'" "For if he says it, it reaches every righteous servant in the heaven and the earth. 'I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and His messenger.' Then he may choose whatever he wishes of the supplication."