Sahih Muslim Book 26, Hadith Number 5389.

Chapter : Prohibition of saying first as-Salam-u-Alaikum to the people of the book, and how their salutations should be responded.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: Do not greet the Jews and the Christians before they greet you and when you meet any one of them on the roads force him to go to the narrowest part of it.