Sunan Abu Dawood, Chapter: Purification (Kitab Al-Taharah)

Sahlah daughter of Suhayl had a prolonged flow of blood. She came to the Prophet (peace_be_upon_him). He commanded her to take a bath for every prayer. When it became hard for her, he commanded her to combine the noon and afternoon prayers with one bath and the sunset and night prayer with one bath, and to take a bath (separately) for the dawn prayer.