Musnad Ahmad 775:10

Ali narrated to us, Mu'adh ibn Hisham narrated to us, my father narrated to me, from al-Qasim ibn 'Awf, a man from the people of Kufa, one of the Banu Murrah ibn Hammam, from 'Abd al-Rahman ibn Abi Layla, from his father, from Mu'adh ibn Jabal, who said: He came to al-Sham and saw the Christians. He mentioned its meaning, except that he said: I said, "For what reason do you do this?" They said, "This was the greeting of the prophets before us." I said, "We are more entitled to do this for our Prophet." The Prophet of Allah (peace and blessings of Allah be upon him) said: "They lied about their prophets just as they distorted their scripture. Indeed, Allah the Almighty has replaced it for us with something better than that: *al-salam* (peace), the greeting of the people of Paradise."