The Book Pertaining to Piety and Softening of Hearts (Kitab Al-Zuhd wa Al-Raqa'iq)

Muslim :: Book 42 : Hadith 7130

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, he should keep his mouth shut with the help of his hand, for it is the devil that enters therein.