It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Allah Almighty likes sneezing and dislikes yawning. Therefore, when a person sneezes and says "Alhamdulillah," it is the right of every listener to say "Yarhamukallah." And when someone yawns, he should try his best to restrain it and not say "Ha Ha," because when a person opens his mouth to yawn and says "Ha Ha," it is Satan who laughs from inside his belly.