Abu Hurairah (Ra) narrates that Nabí (saw) said: Indeed, Allãh likes sneezing and dislikes yawning. So, when one of you sneezes, he should say Alhamdulillãh (Praise be to Allãh). It is right on every Muslim who hears him to say Yarhamukallãh (may Allãh have mercy on you) in reply. Indeed, yawning comes from Shaitan. So when one of you yawns, he should restrain himself as much as possible, as Shaitãn laughs at the person who yawns.
(Bukhãrí)