Abu Huraira (may Allah be pleased with him) narrates from the Prophet (peace and blessings of Allah be upon him), he said: "Indeed, Allah likes sneezing and dislikes yawning. When one of you sneezes and says 'Alhamdulillah' (All praise is due to Allah), it is obligatory upon every Muslim who hears him to say to him 'Yarhamukallah' (May Allah have mercy on you). As for yawning, it is from the devil; when one of you yawns, he should try his best to restrain it, for when one of you yawns, the devil laughs at him." (Bukhari) And in the narration of Sahih Muslim: "Because when one of you makes a sound while yawning, the devil laughs at him." Narrated by al-Bukhari and Muslim.