Sahlah bint Suhayl experienced prolonged menstrual bleeding, so she came to the Prophet ﷺ, who instructed her to perform a full bath before every prayer. When this became too difficult for her, he instructed her to combine Zuhr and Asr with one bath, Maghrib and Isha with another bath, and to perform a separate bath for Fajr.
حدثنا عبد العزيز بن يحيى حدثني محمد بن سلمة عن محمد بن إسحق عن عبد الرحمن بن القاسم عن أبيه
أن سهلة بنت سهيل استحيضت فأتت النبي ﷺ فأمرها أن تغتسل عند كل صلاة فلما جهدها ذلك أمرها أن تجمع بين الظهر والعصر بغسل والمغرب والعشاء بغسل وتغتسل للصبح
Sourcing, [its chain of transmission is daif]. Bayhaqi reported it, 1/352-353, from the hadith of Abu Dawud through this route. See the preceding hadith and the hadith of Ibn Uyayna, which Bayhaqi reported, 1/353. Ibn Ishaq and Sufyan are both mudallis narrators and they narrated using "an" (indicating possible concealment in the chain).