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.”
Bukhari Vol. 8 : No. 242
