Sunan An-Nasa'i
Hadith #354
It was narrated that Umm Salamah said:""A woman asked the Prophet (ﷺ): 'I suffer from Istihadah and I never become pure; should I stop praying?' He said: 'No. Stop praying for the number of days and nights that you used to menstruate, then perform Ghusl, wrap a cloth around yourself, and pray