النهي عن النفخ في الإناء
A prohibition on breathing into a cup while drinking from it or blowing directly onto food or drink to cool it.
نَهَى أَنْ يُتَنَفَّسَ فِي الْإِنَاءِ أَوْ يُنْفَخَ فِيهِ
“He forbade breathing into a vessel [while drinking] or blowing into it.” (meaning, not a literal translation)
Commentators read this straightforwardly as hygiene — avoiding the transfer of saliva and breath-borne particles into shared food and drink, a principle modern food safety fully endorses.