Yahya ibn Yahya narrated to us, Hushaim informed us, from Ubaydullah ibn Abi Bakr, who said: I heard Anas say: The Messenger of Allah, peace and blessings be upon him, said...
And Isma'il ibn Salim narrated to me, Hushaim narrated to us, Ubaydullah ibn Abi Bakr informed us, from his grandfather Anas ibn Malik, that the Messenger of Allah, peace and blessings be upon him, said:
"If the People of the Book greet you, then say: 'And upon you.'"

Anas reported that the Companions of Allah's Apostle ( صلی اللہ علیہ وسلم ) said to him: The People of the Book offer us salutations (by saying as-Salamu-'Alaikum). How should we reciprocate them? Thereupon he said: Say: Wa 'Alaikum (and upon you too).