Sahih Muslim, Chapter: Prayers (Kitab Al-Salat)

While observing prayer behind the Messenger of Allah (may peace be upon him) we used to recite: Peace be upon Allah, peace be upon so and so. One day the Messenger of Allah (may peace be upon him) said to us: Verily Allah is Himself Peace. When any one of you sits during the prayer. he should say: All services rendered by words, by acts of worship, and all good things are due to Allah. Peace be upon you,0 Prophet, and Allah's mercy and blessings. Peace be upon us and upon Allah's upright servants, for when he says this it reaches every upright servant in heaven and earth (and say further): I testify that there is no god but Allah and I testify that Muhammad is His servant and Messenger. Then he may choose any supplication which pleases him and offer it.