Musnad Ahmad 32:201

Yahya narrated to us, from Shu'bah, Qatadah narrated to us, and Ibn Ja'far said: Shu'bah narrated to us, he said: I heard Qatadah, from Anas, who said: The companions of the Prophet asked the Prophet, peace and blessings of Allah be upon him, and said: "The People of the Scripture greet us, so how should we respond to them?" He said: "Say: 'Wa 'alaykum' (And upon you)." Hajjaj said the like of it. Shu'bah said: I did not ask Qatadah about this hadith, whether he heard it from Anas.