Musannaf Ibn Abi Shaybah 39395 — Reports on the letters of the Prophet

Abu Burdah said that the Messenger of Allah (peace and blessings be upon him) wrote a letter to a man from the People of the Book: "Accept Islam." The narrator said: the Messenger of Allah (peace and blessings be upon him) had not yet finished (dictating) his letter when a letter from that same man came to him, in which the man was invoking peace upon him (peace and blessings be upon him). So he (peace and blessings be upon him) returned his greeting of peace at the end of his letter.