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 the Prophet (peace and blessings be upon him) ordered her: "She should delay Zuhr and hasten Asr and perform one ritual bath (ghusl) for both of them; and as for Maghrib and Isha, she should hasten this one and delay that one, and perform one ghusl for both of them."
This is an error on the part of al-Hasan (bin Sahl).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1652