Narrated Abu Huraira: I heard Allah's Messenger (ﷺ) saying, "The rights of a Muslim on the Muslims are five: to respond to the salaam, visiting the sick, to follow the funeral processions, to accept an invitation, and to return the greeting and to reply to the sneezer: (saying, "May Allah be merciful on you," provided the sneezer says, "All the praises are for Allah,").

Reference: Bukhari: 1240