Riyad us Saliheen: Hadith 895

The Messenger of Allah (ﷺ) said, "Every Muslim has five rights over another Muslim (i.e., he has to perform five duties for another Muslim): to return the greetings, to visit the sick, to accompany funeral processions, to accept an invitation, to respond to the sneezer

[i.e., to say: 'Yarhamuk-Allah (may Allah bestow His Mercy on you),' when the sneezer praises Allah]."

[Al-Bukhari and Muslim].