آداب التثاؤب
Guidance to suppress a yawn where possible and to cover the mouth with the hand when one occurs.
إِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيُمْسِكْ بِيَدِهِ عَلَى فِيهِ
“When one of you yawns, let him place his hand over his mouth.” (meaning, not a literal translation)
The fuller hadith describes yawning as something Allah dislikes, in contrast to sneezing, which is described favorably — the same narration states that Satan laughs at an uncovered yawn, which is part of why the etiquette here is to restrain and cover rather than simply cover, unlike the sneezing guidance which doesn't discourage the act itself.
Paired in the same hadith with a description of yawning as undesirable and best restrained where possible — commentators note the shared hygiene logic with sneezing etiquette. Ibn al-Qayyim discusses the contrast between the two: sneezing is linked in the hadith corpus to relief and even divine favor, while yawning is associated with sluggishness and is something to actively work against, reflecting a broader theme in the tradition of associating alertness with virtue.
Covering the mouth during a yawn serves the same droplet-reduction hygiene purpose modern public health guidance recommends for coughs and sneezes, even though yawning itself is understood today as a normal physiological response (linked to brain temperature regulation and alertness cycles) rather than something to suppress for health reasons.