Sunan an-Nasa'i 201

It was narrated from Fatimah bint Qais from Banu Asad Quraish that she came to the Prophet (ﷺ) and mentioned that she suffered from Istihadah (non-menstrual vaginal bleeding). She said that he said to her:"That is (bleeding from) a vein, so when the time of menstruation comes, stop praying, and when it goes, then wash the blood from yourself then pray