It was narrated from 'Aishah that the daughter 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.