(3) باب الْمَرْأَةِ يَكُونُ لَهَا أَيَّامٌ مَعْلُومَةٌ تَحِيضُهَا كُلَّ شَهْرٍ
It was narrated from Umm Salamah that a woman suffered from constant bleeding during the time of the Messenger of Allah (ﷺ), so Umm Salamah consulted the Prophet (ﷺ) for her. He said:
"Let her count the number of nights and days that she used to menstruate each month before this happened to her, and let her stop praying for that period of time each month. Then when that is over let her perform Ghusl, then wrap a cloth around herself, and pray."
Reference: Sunan an-Nasa'i 355
In-book reference: Book 3, Hadith 7
English translation: Vol. 1, Book 3, Hadith 355