Shaikh Ibn Baz rahimahullah was asked:

Question:

Is it allowed for the one praying to place his hand on his mouth if he yawns during prayer?

Answer:

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….

[Reference]

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan