Narrated Abu Huraira (Radi-Allahu 'anhu):

The Prophet (Sallallahu 'Alaihi Wa Sallam) 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."