Abu Dawood: Hadith 284

Bahiyyah said: I heard a woman asking Aisha about the woman whose menses became abnormal and she had an issue of blood. The Messenger of Allah (ﷺ) asked me to advise her that she should consider the period during which she used to menstruate every month, when her menstruation was normal. Then she should count the days equal to the length of time (of her normal menses); then she should abandon prayer during those days or equal to that period. She should then take a bath, tie a cloth on her private parts a pray.