Q: What is the ruling on covering the mouth and nose in prayer?
A: It is disliked (makrūh) to cover the face entirely or to cover the mouth and nose during prayer. This is based on the ḥadīth which states, “The Prophet prohibited that a man covers his mouth” (Abu Dawud). This ḥadīth, though explicit regarding the mouth, includes the face because the mouth would be covered if the face was. It is also used to make qiyas for the nose.
Covering the face resembles the state of the majus during their worship of the fire and could potentially prevent one from properly reciting the letters in prayer.
It should be noted that doing something makrūh in prayer does not invalidate it. Also, as a principle, needs permit what is disliked just as necessity knows no law.
Source: Sharh al-Muntah