I say, and with God's success: If his mouth becomes impure, and he drinks water immediately, then his leftover (the water he drank) is impure. However, if he has saliva in his mouth several times and spits it out or swallows it before drinking, then his leftover is not impure. There is no difference in the ruling of the leftover of a human being between the adult and the child, the Muslim and the non-Muslim, the menstruating woman and the one in a state of major impurity; as narrated by Aisha (may Allah be pleased with her), she said: "I used to drink while I was menstruating, then I would hand it to the Prophet (peace be upon him) and he would place his mouth on the spot where I drank from it and drink." This is found in Sahih Muslim 1: 245, Sahih Ibn Khuzaymah 1: 58, and Sahih Ibn Hibban 4: 108. Refer to: Maraqi al-Falah, p. 29, and Allah knows best.