Hadith Bukhari 4:509, Narrated Abu Huraira:
The Prophet (peace be upon him) 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.
Hadith Bukhari 8:242, 8:245, Narrated Abu Huraira
The Prophet (peace be upon him) 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 ones best to stop it, if one says Ha when yawning, Satan will laugh at him.
Hadith Muslim, Narrated AbuSaid al-Khudri
Allahs Apostle (peace be upon him) said: When one of you yawns, he should try to restrain it with the help of his hand since it is the Satan that enters therein.
Hadith Mishkat, Narrated AbuHurayrah , transmitted by Tirmidhi and Ibn Majah.
Allahs Messenger (peace be upon him) said, Yawning in prayer is an act of Shaytan, so when one of you yawns he should restrain it as much as possible. In another version it the word are: He should place his hand upon his mouth.