Sunan An-Nasai, Chapter: The Book of Purification

Fatimah bint Abi Hubaish came to the Messenger of Allah (saw) and said: "I am a woman who suffers from Istihadah (non-menstrual vaginal bleeding) and I never become pure. Should I stop praying?" He said: "No, that is a vein, it is not menstruation. When your period comes, stop praying, and when it goes, wash the blood from yourself and pray." (Sahih)