Good Manners and Form (Al-Adab)
Bukhari :: Book 8 :: Volume 73 :: Hadith 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."
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