And performing ablution and ghusl with it is permissible because it is pure, not impure, because if the leftover water of a menstruating woman was impure, then the Prophet (peace be upon him) would not have drunk the impure water, especially when he (peace be upon him) was not even compelled to drink it.

Sayyidah Aisha (may Allah be pleased with her) narrates that when a (drink) vessel was brought to the Messenger of Allah (peace and blessings be upon him), I would begin drinking from it even though I was menstruating. Then he would take the vessel and place his mouth where I had placed mine (and drink from it). And when (eating food) I would take a bone and gnaw meat from it—then (the Messenger of Allah would take that bone and) place his mouth where I had placed mine (and eat from it). The Imam (author) says: Salm bin Junadah has also narrated a similar report to us with this same chain.