Sahih Al Bukhari – Book of Beginning Of Creation
Volumn 004, Book 054, Hadith Number 509.
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Narated By Abu Huraira : The Prophet said, "Yawning is from Satan and if anyone of you yawns, he should check his yawning as much as possible, for if anyone of you (during the act of yawning) should say: 'Ha', Satan will laugh at him."
Junaid Tahir