Musnad Abu Ya'la al-Mawsili 155:94

From Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), he said: "Among the rights of a Muslim upon a Muslim are: witnessing the funeral, returning the greeting, visiting the sick, saying 'Yarhamuk-Allah' (may Allah have mercy on you) to the sneezer if he praises Allah, and accepting the invitation."