Musnad Abu Ya'la al-Mawsili 155:64

Narrated to us Shayban, narrated to us Abu 'Awanah, from 'Umar ibn Abi Salamah, from his father, from Abu Hurairah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Three things are all a right upon the Muslim: visiting the sick, attending funerals, and saying 'Yarhamuk Allah' (may Allah have mercy on you) to the one who sneezes if he praises Allah."