Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 245
Narrated Abu Huraira:
The Prophet said, "Allah loves sneezing but dislikes yawning; so if anyone of you sneezes and then praises Allah, every Muslim who hears him (praising Allah) has to say Tashmit to him. But as regards yawning, it is from Satan, so if one of you yawns, he should try his best to stop it, for when anyone of you yawns, Satan laughs at him."