Sahih Muslim
Sahih Muslim Book 026, Hadith Number 5380.
Hadith Section/Book: Sahih Muslim Sahih Muslim Book 26. On Salutations and Greetings
Chapter : Prohibition of saying first as-Salam-u-Alaikum to the people of the book, and how their salutations should be responded.
Anas b. Malik reported Allah's Messenger (may peace be upon him) as saying: When the People of the Book offer you salutations, you should say: The same to you.
Anas b. Malik reported Allah's Messenger (may peace be upon him) as saying: When the People of the Book offer you salutations, you should say: The same to you.