Musnad Ahmad 1237:12

Abu Asim narrated to us, from Abd al-Hamid—meaning Ibn Ja'far—who said: Yazid ibn Abi Habib informed me, from Marthad ibn Abdullah, from Abu Basra al-Ghifari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to them one day: "I am riding to the Jews, so whoever goes with me, if they greet you, then say: 'And upon you.'" So we set out, and when we came to them and they greeted us, we said: "And upon you."