Musannaf Ibn Abi Shayba 3:2883

Ghundar narrated to us, from Shu'bah, from al-Hakam and Hammad, regarding the man who enters the mosque on Friday while the Imam has already come out: He said: He greets them, and they return the greeting to him. And if he sneezes, they say "Yarhamuk Allah" (may Allah have mercy on you) to him, and they return the greeting to him.