Musnad Ahmad 30:3666

Yahya ibn Adam narrated to us, Sufyan narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "When you meet the polytheists on the road, do not initiate the greeting of peace with them, and force them to the narrowest part of it."