Shaikh Ibn Baz rahimahullah was asked:
Is it allowed for the one praying to place his hand on his mouth if he yawns during prayer?
This is the Sunnah. Yes, he places his hand on his mouth in prayer and outside prayer also. He should not say: Haa, but supress his voice, cover his mouth, and place his hand on his mouth….
Faisal Ibn Abdul Qaadir Ibn Hassan