(a) It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Umm Habibah bint Jahsh (may Allah be pleased with her) asked the Messenger of Allah (peace and blessings be upon him): I have istihadah. He (peace and blessings be upon him) said: "That is a vein, so perform ghusl and pray." She used to perform ghusl for every prayer. Al-Layth said: Ibn Shihab did not mention that the Messenger of Allah (peace and blessings be upon him) ordered Umm Habibah (may Allah be pleased with her) to perform ghusl for every prayer; rather, it was something she did herself.
(b) It is narrated from Imam al-Shafi'i (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) ordered her to perform ghusl and pray; it does not contain that he (peace and blessings be upon him) ordered her to perform ghusl for every prayer. In sha' Allah (if Allah wills), I have no doubt that her ghusl was voluntary; it had not been made obligatory upon her.
(c) Imam al-Shafi'i (may Allah have mercy on him) said: Others besides al-Zuhri narrated this hadith, (stating) that the Prophet (peace and blessings be upon him) ordered her to perform ghusl for every prayer. But they narrated it from Amrah with this chain and context, and al-Zuhri's narration is better preserved than it. In it something is stated that indicates the hadith is mistaken: that she will leave prayer for the extent of her menstruation. Sayyidah Aishah (may Allah be pleased with her) says that al-aqra' means the periods of purity.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1639