It is narrated from Sayyiduna Abu Basrah al-Ghifari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to the Companions: "I am riding to the Jews. Whoever of you goes with me, let him not initiate the greeting of salam with them; if they greet you, then say only: 'Wa alaykum (And upon you).'" When we reached them, they greeted us, and we said: "Wa alaykum."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2149