Musnad Ahmad 30:1278

Muhammad ibn Bishr narrated to us, Muhammad ibn ‘Amr narrated to us, Abu Salamah narrated to us, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Five are the rights of a Muslim upon a Muslim: returning the greeting, accepting the invitation, attending the funeral, visiting the sick, and saying 'Yarhamuk Allah' (may Allah have mercy on you) to the one who sneezes if he praises Allah."