Musannaf Ibn Abi Shayba 28:1016

Waki’ narrated to us, from Sufyan, from Mansur, who said: I asked Ibrahim and Mujahid, "How should one write to the People of the Covenant (Ahl al-Dhimmah)?" Mujahid said: "One writes, 'Peace be upon those who follow guidance.'" Ibrahim said: "Peace be upon you."