Musnad Ahmad 30:449

Abu Kamil narrated to us, Zuhayr narrated to us, Suhayl ibn Abi Salih narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If you meet them on a road, do not initiate the greeting with them, and force them to its narrowest part." Zuhayr said: So I said to Suhayl: "The Jews and the Christians?" He said: "The polytheists."