The Messenger of Allah ﷺ said, "When one of you yawns, let him cover his mouth with his hand, for Satan enters through it." He also said that whoever yawns during prayer should stifle it as much as he can.
حدثنا ابن العلاء عن وكيع عن سفيان عن سهيل نحوه قال في الصلاة فليكظم ما استطاع