Bulugh al-Maram #168

Narrated Hamnah bint Jahsh: 'I had a very strong prolonged flow of blood. I went to the Prophet (Peace be upon him) to ask him about it. He said, "This is a strike from Satan. So observe your menses for six or seven days, then perform Ghusl until you see that you are clean. Pray for twenty-four or twenty-three nights and days and fast, and that will suffice you. Do so every month just as the other women menstruate (and are purified). But if you are strong enough to delay the Dhuhr prayer and advance the Asr prayer, then make Ghusl when your purified and combine the Dhuhr and the Asr prayers together; then delay the Maghrib prayer and advance the Isha prayer, and perform Ghusl and combine the two prayers, do so. Do so, and then wash at dawn and pray Fajr. This is how you may pray and fast if you have the ability to do so." And he said, "That is the more preferable way to me."