Covering the mouth when yawning
The Prophet (sal Allahu alaihi wa sallam) said, “When one yawns, he should put his hand over his mouth, otherwise the devil will enter.”
We benefit from the above Hadith that at the time of yawning, one should put his hand across his mouth. It is a disliked act (mukruh) to yawn loudly, because this act pleases Satan. In other words, we must not miss any chance to degrade and frustrate Satan.
And Allah knows best!