Musnad Ahmad 30:1911

'Affan narrated to us, Abu 'Awanah narrated to us, 'Umar ibn Abi Salamah narrated to us, from his father, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Three things are all 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."