From Abu Sa’eed who said, “The Messenger of Allaah صلى الله عليه وسلم forbade drinking from the cracked place on a cup …” [Abu Dawud, no. 3722]
Al-Albaani said, “Sahih li ghairihi.” Then he quoted Ibn al-Athir’s explanation of the term, ‘ثُلْمَة – cracked place,’ “I.e., the place where it is broken, and he only forbade it because the mouth of the one drinking cannot grasp [the cup] properly and [thus] the water may spill onto his garments or body. And it has been said: [it was said] because when the cup is washed, the place where it is cracked cannot be cleaned completely.” An-Nihaayah.
I [al-Albaani] say, “And today it is possible to say: [it was said] because it is the place where germs and microbes gather, so it is from Prophetic medicine, and a scientific miracle.
So may Allaah send prayers upon the Prophet, the Unlettered one.”
Saheeh Mawaarid adh-Dhamaan, 2/15.