Musannaf Ibn Abi Shayba 28:1017

Waki’ narrated to us, from ‘Amr ibn ‘Uthman, from Abu Burdah, who said: I heard him saying: The Messenger of Allah (may Allah bless him and grant him peace) wrote to a man from the People of the Scripture: "Submit (embrace Islam), and you will be safe." The Prophet (may Allah bless him and grant him peace) had not yet finished his letter when a letter came to him from that man, in which he greeted the Prophet (may Allah bless him and grant him peace) with peace. So the Prophet (may Allah bless him and grant him peace) returned the greeting of peace at the bottom of his letter.