It is narrated from Sayyidah Zaynab bint Jahsh (may Allah be pleased with her) that she said: I asked the Messenger of Allah (peace and blessings be upon him) about Hamnah (may Allah be pleased with her), saying that she has istihadah (irregular bleeding). He (peace and blessings be upon him) said: "She should refrain (from prayer) during the days of her menstruation, then perform a ritual bath (ghusl); and she should delay Zuhr and pray Asr early, then perform ghusl and pray; and delay Maghrib and pray Isha early, and perform ghusl and pray them both; and perform one ghusl for Fajr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1657