Sunan An-Nasai, Chapter: The Book of Menstruation and Istihadah

that the daughte of Abu Hubaish said: "O Messenger of Allah, I do not become pure, so should I stop praying?" He said: "No, that is a vein." - (One of the narrators) Khalid said, in what I read from him - "and it is not menstruation, so when your period comes, stop praying, and when it goes, wash the blood from yourself and pray." (Sahih)