Musnad Ahmad 1153:862

Ahmad ibn Abd al-Malik narrated to us, Muhammad ibn Salamah narrated to us, from Muhammad ibn Ishaq, from Abd al-Rahman ibn al-Qasim, from his father, from Aisha, who said: "Sahlah bint Suhayl ibn Amr experienced *istihada* (prolonged non-menstrual bleeding), so she came to the Messenger of Allah, peace and blessings of Allah be upon him, and asked him about that. He commanded her to perform *ghusl* (full ablution) for every prayer. When that became difficult for her, he commanded her to combine the Dhuhr and Asr prayers with one *ghusl*, the Maghrib and Isha prayers with one *ghusl*, and the Subh (Fajr) prayer with one *ghusl*."