Sahih Muslim Book 26, Hadith Number 5390.

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

This hadith has been narrated on the authority of Suhail with the same chain of transmitters but with a slight variation of wording. The hadith transmitted on the authority of Waki’, the words are ‘When you meet the Jews.” And in the hadith transmitted on the authority of Shu’ba, the words are: ‘When you meet the People of the Book.” And in the hadith transmitted on the authority of Jarir the words are: “When you meet them,” but none amongst the polytheists has been mentioned explicitly by name.