What is the evidence that a small amount of whatever prevents water from reaching the skin is excused? And is the dirt that forms at the start of the anal ring excused?

Scholars have differed regarding minor impediments that prevent water from reaching a limb. Some have not excused them at all, while others have excused slight dirt under the fingernail due to the difficulty of avoiding it. Sheikh al-Islam extended this ruling to include any slight dirt anywhere on the body, such as blood or dough, by analogy with what is under the fingernail. He mentioned in Sharh al-Iqna' that slight dirt under the fingernail does not harm, even if it prevents water from reaching the skin, because it is something that frequently occurs, and if it nullified ablution, the Prophet (peace and blessings of Allah be upon him) would have clarified it.

Based on this, if the dirt in question originates from the body, it does not cause harm and is not considered an impediment. However, if it does not originate from the body, then the aforementioned difference of opinion applies: Sheikh al-Islam considers it excused if it is minor, while the majority of scholars do not. The view of the majority is more cautious.