Mukhtasar Bukhari

Narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings of Allah be upon him) said: "Allah likes sneezing and dislikes yawning. When someone sneezes and says 'Alhamdulillah', it becomes obligatory upon every Muslim who hears him to say 'Yarhamukallah'. But yawning is from Satan, so one should try to stop it as much as possible, because when a person yawns and says 'Ha', Satan laughs at him."