Sahih Muslim Book 42, Hadith Number 7132.

Chapter : Sneezing and the disapproval of yawning.

The son of Abu Said al-Khudri reported on the authority of his father that Allah’s Messenger (may peace be upon him) said. When one of you yawns while engaged in prayer, he should try to restrain so far as it lies in his power, since it is the Satan that enter therein.