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)
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.