غسل اليدين عند الاستيقاظ
Washing the hands before dipping them into a shared water vessel upon waking from sleep.
إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلْيَغْسِلْ يَدَيْهِ قَبْلَ أَنْ يُدْخِلَهُمَا فِي الْإِنَاءِ، فَإِنَّ أَحَدَكُمْ لَا يَدْرِي أَيْنَ بَاتَتْ يَدُهُ
“When one of you wakes from sleep, let him wash his hands before putting them into a vessel, for none of you knows where his hand spent the night.” (meaning, not a literal translation)
A directly hygienic instruction, cited by modern commentators as a striking early parallel to handwashing protocols in contemporary infection control.