Musannaf Ibn Abi Shaybah

Hazrat Mansur (may Allah have mercy on him) says that I asked Ibrahim (may Allah have mercy on him) and Hazrat Mujahid (may Allah have mercy on him) how a letter should be written to the dhimmis. Hazrat Mujahid said: It should be written to them: "As-salamu 'ala man ittaba'a al-huda" (Peace be upon him who follows guidance), and Ibrahim said: Write "Salam 'alaik" (Peace be upon you).