Volume 8, Book 73, Number 242:
Narrated Abu Huraira:
The Prophet said, “Allah likes sneezing and dislikes yawning, so if someone sneezes and then
praises Allah, then it is obligatory on every Muslim who heard him, to say: May Allah be merciful to
you (Yar-hamuka-l-lah). But as regards yawning, it is from Satan, so one must try one’s best to stop
it, if one says ‘Ha’ when yawning, Satan will laugh at him.”