Musnad Ahmad 30:2791

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said regarding the People of the Book: "Do not initiate the greeting of peace (salam) with them, and if you meet them on a road, then force them to its narrowest part."