Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.012.

Jallalah refers to an animal that eats the waste or flesh of other animals, such as camels, cows, sheep, chickens, geese, and so on. Ibn ‘Abbas reported that the Messenger of Allah forbade the drinking of such animals’ milk.

This hadith is related by “the five,” except for Ibn Majah. At-Tirmizhi grades it as sahih. In one narration it states, “It is also prohibited to ride upon a jallalah. (Related by Abu Dawud.) ‘Amr ibn Shu’aib related on the authority of his father, from his grandfather, that the Messenger of Allah prohibited the meat of domestic donkeys. As for the jallalah, he prohibited riding or eating them.” (Related by Ahamad, an-Nasa’i and Abu Dawud.) If the jallalah animal is kept away from the other animals for some time and is given clean food to eat, then it becomes pure and is no longer called jallalah. If this is the case, it becomes permissible to eat, as the reason for its prohibition was the change it underwent due to eating filth, a state which would no longer be present.