Musnad Ahmad 30:499

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not initiate the greeting of peace (salam) to the Jews and the Christians. And if you meet them on a road, force them to its narrowest part."