Musannaf Ibn Abi Shayba 17:448

Yazid ibn Harun narrated to us, he said: Al-Mas'udi informed us, from 'Awn ibn 'Abd Allah, he said: Muhammad ibn Ka'b asked 'Umar ibn 'Abd al-'Aziz about initiating the greeting of peace (salam) to the People of the Covenant (Ahl al-Dhimma). He said: "We return it to them, but we do not initiate it." I said: "And what do you say yourself?" He said: "I do not see any harm in us initiating it for them." I said: "Why?" He said: "Because of the saying of Allah: 'So pardon them and say, "Peace." But they are going to know.'"