Sahih Muslim 2011
Sahih Muslim · Book 36: The Book of Drinks · Drinking Nabidh and covering vessels
Jabir reported that a person who was known as Abu Humaid brought for him (the Holy Prophet) a cup of milk from al-Naqi'. Allah's Messenger (ﷺ) said to him:
وَحَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا جَرِيرٌ ، عَنِ الأَعْمَشِ ، عَنْ أَبِي سُفْيَانَ ، وَأَبِي، صَالِحٍ عَنْ جَابِرٍ ، قَالَ جَاءَ رَجُلٌ يُقَالُ لَهُ أَبُو حُمَيْدٍ بِقَدَحٍ مِنْ لَبَنٍ مِنَ النَّقِيعِ فَقَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَلاَّ خَمَّرْتَهُ وَلَوْ تَعْرُضُ عَلَيْهِ عُودًا " .
Why did you not cover it even with a wood across it?