When the menstruation begins, you should abandon prayer; when the period equal to its length of time passes, you should wash away the blood and pray.
Sunan Abi Dawud Book 1, Hadith 283
← Previous Hadith Fatimah daughter of Abu Hubaish came to the Messenger of Allah (ﷺ) and said: I am a woman who has prolonged flow of blood; I am never purified ; should I abandon prayer ? He replied: This is (due to) a vein, and not menstruation. When the menstruation begins, you should abandon prayer ; when it is finished, you should wash away the blood and pray.

Sunan Abi Dawud Book 1, Hadith 282 Next Hadith → 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.

Sunan Abi Dawud Book 1, Hadith 284