When a person yawns, he should block his mouth with his hand as Shaytaan enters, i.e. the mouth.
(Sahih Muslim vol.2 pg.413; Rashidiyyah)
When a person sneezes and praises Allah, it is a right upon every Muslim who hears him that he replies to him and as for yawning, it is from Shaytaan. So, he should try to stop yawning to the best of his ability. When a person yawns results in a sound haa, Shaytaan laughs at him.
(Bukhari vol.2 pg.919; Qadeemi)