It is narrated from Sayyidah Aishah (may Allah be pleased with her) that a woman began to have istihadah (irregular bleeding) in the time of the Prophet (peace and blessings be upon him), and she was ordered: "She should delay Zuhr and hasten Asr and perform one ritual bath (ghusl) for both of them; and delay Maghrib and pray Isha early and perform one ghusl for both of them; and perform one ghusl for the morning prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1653