Musnad Ahmad 32:1937

Muhammad ibn Ja'far narrated to us, Shu'bah and al-Hajjaj narrated to us, he said: Shu'bah narrated to me, he said: I heard Qatadah narrating from Anas ibn Malik. And Yahya ibn Sa'id narrated to us, from Shu'bah, he said: Qatadah narrated to us, from Anas, and the meaning is the same: That the companions of the Prophet, peace and blessings of Allah be upon him, said to the Prophet, peace and blessings of Allah be upon him: "The People of the Book greet us, so how should we respond to them?" He said: "Say: 'Wa 'alaykum' (And upon you)." And Hajjaj said: Shu'bah said: "I did not ask Qatadah about this hadith: 'Did you hear it from Anas?'"