may Allah have mercy on you,’ that is sufficient; yet, to err on the side of caution, every person who hears the sneeze should offer the tashmīt—invoking mercy for him—as the ḥadith instructs.
Yawning, on the other hand, is from Satan, and for that reason Allah detests it. Why? Because yawning signals laziness; it increases when a person is drowsy, and since it betrays lethargy Allah dislikes it. When you feel a yawn coming on, the preferred course is to repel it: suppress the yawn and exercise self-restraint. The scholars say that if you wish to stifle it, press your lower lip lightly—not so hard that you injure it, but just enough to keep the mouth from opening. The essential point is to suppress the yawn, whether by this means or another. If you cannot do so, place your hand over your mouth. What some scholars have mentioned—that you should cover the mouth with the back of the hand—has no foundation; rather, use the palm so that the mouth is fully covered.
The reason is that when a person yawns, Satan laughs at him, for he recognizes yawning as a sign of indolence and sluggishness. Satan loves to see the children of Adam idle and lethargic—may Allah protect us and you from him—and he despises the energetic, resolute person who is always disciplined, strong, and active. Therefore, whenever a yawn overtakes you, if you are able to suppress and prevent it, that is the Sunnah and the better course. If you cannot, then place your hand over your mouth.