Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Yawning inside the prayer is from the devil, so if anyone of you yawns, let him suppress it as much as he can."
Yawning inside the prayer is from the devil, because it occurs when the body is heavy, relaxed, full of food, and when the person is feeling lax and sleepy. It is the devil who calls for giving the self whatever it desires and supplying it with abundant food and drink. Therefore, when someone who is offering the prayer begins to yawn, or feels like yawning, he should suppress it as much as possible, by shutting his lips and teeth tight. He should do this so that the devil would not accomplish his aim of distorting the image of the praying person, entering his mouth, and laughing at him. If he cannot suppress yawning, then he should place his hand over his mouth.