Sunan An-Nasai, Chapter: The Book of Purification

that a woman who suffered from Istihadah during the time of the Messenger of Allah (saw) was told that it was a stubborn vein (i.e., one that would not stop bleeding). She was told to delay Zuhr and bring Asr forward, and to perform one Ghusl for both, and to delay Maghrib and bring Isha forward, and to perform one Ghusl for both, and to perform one Ghusl for Subh. (Sahih)