Musnad Ahmad 30:1554

Yahya ibn Ishaq narrated to us, Abu 'Awanah narrated to us, from 'Umar ibn Abi Salamah, from his father, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Three things, all of them are a duty upon every Muslim: visiting the sick, attending the funeral, and saying 'Yarhamuk Allah' (may Allah have mercy on you) to the one who sneezes, if he praises Allah, the Almighty and Majestic."