The well-known view of the madhhab is that if it is substantial—whether by customary practice or according to each individual’s own assessment, in keeping with the earlier disagreement—it invalidates ablution; but if it is slight, it does not.
They cited the following evidence for this:
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The Prophet ﷺ vomited, broke his fast, and performed ablution. (1) Allah, Exalted is He, says: “Indeed, in the Messenger of Allah you have an excellent example.” [al-Ahzab 33:21] Since he performed ablution after vomiting, following his excellent example means that we should do as he did.
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These are bodily wastes that emerge from the body, and so they resemble urine and feces. Yet they do not share their ruling in every respect, because the point from which they emerge differs. Thus, they are given the same ruling in one respect but not another: a small or large amount of urine and feces invalidates ablution because it exits from the two passages, whereas other substances invalidate it only when substantial.
Al-Shafi‘i and the seven jurists (2)—who are gathered together in the verse composed by one of them—held otherwise:
When it is asked, “Who are the seven seas of knowledge,
Whose narrations lie not beyond the bounds of knowledge?”