According to the motherhood rule (al-'Ibrah li-l-Umm), under which condition is a hybrid mule's leftover classified as doubtful (Mashkuk)?

Leftover Water (Su'r): The Four Legal Categories

• Only when its mother was a female donkey (Atān).
• Only when its mother was a mare (female horse).
• Mules' leftovers are always pure and purifying.
• Mules' leftovers are always impure.