Abu Hurayrah (may Allah be pleased with him) reported: I heard the Messenger of Allah (may Allah’s peace and blessings be upon him) say: "A Muslim owes another Muslim five rights: returning the greeting of peace, visiting the sick, following the funeral processions, accepting invitations, and saying tashmīt (yarhamuk Allah) (may Allah show mercy to you) to the person who sneezes."
The Prophet (may Allah's peace and blessings be upon him) mentions some of the rights Muslims have toward one another. The first among these rights is to return the greeting of peace to the one who greets you. The second right is to visit the sick. The third right is to follow the funeral procession from the deceased person's home to the praying place and the cemetery until he is buried. The fourth right is to accept the invitation of someone who invites you to a wedding banquet and the like. The fifth right is to say tashmīt to a person who sneezes and praises Allah. Tashmīt is to say: yarhamuk Allah (may Allah show mercy to you). In response, the sneezing person should say: yahdīkum Allah wa yuslih bālakum (may Allah guide you and amend your condition).