Is the leftover water of a human being (sūr) ritually pure, absolutely, whether the person is a Muslim or a disbeliever, and what is the evidence for this?

The leftover water (su'r) of a human being is pure, whether the person is a Muslim or a disbeliever, according to the majority of scholars, based on the hadith: "The believer does not become impure." It is also authentically reported that the Prophet, peace and blessings be upon him, used to drink from the place where Aishah's mouth was, even while she was menstruating. However, there is a narration from Imam Ahmad stating the impurity of the disbeliever's leftover water. Scholars have differed concerning the leftover water of a wine drinker.