آداب العطاس
Covering the mouth and lowering the voice when sneezing, paired with a specific verbal exchange between the sneezer and those nearby.
كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا عَطَسَ وَضَعَ يَدَهُ أَوْ ثَوْبَهُ عَلَى فِيهِ، وَغَضَّ بِهَا صَوْتَهُ
“When the Messenger of Allah ﷺ sneezed, he would place his hand or garment over his mouth and lower his voice with it.” (meaning, not a literal translation)
The well-known verbal exchange — saying 'al-hamdu lillah' and receiving 'yarhamuk Allah' in reply — is separately recorded in Sahih al-Bukhari 6224, which also notes that the reply is only owed if the sneezer said 'al-hamdu lillah' aloud; if they didn't, no response is expected, tying the social obligation directly to the sneezer's own words rather than the act of sneezing itself.
Commentators note the direct respiratory-hygiene value of covering the mouth, alongside the social and spiritual dimension of the reply exchange. Ibn al-Qayyim discusses sneezing itself favorably in Zad al-Ma'ad — as a natural release of excess in the head thought beneficial in the classical humoral framework — which is part of why the etiquette focuses on how to sneeze considerately, rather than suppressing it, in contrast to the guidance given for yawning.
Modern public health guidance strongly emphasizes covering a sneeze to reduce airborne droplet transmission of respiratory illness — the same underlying principle as this hadith, arrived at independently through germ theory rather than the classical humoral framework it originally sat within.