Scholarly disagreement
Al-Sharḥ al-Kabīr states: “A woman is not required to undo her hair when performing the ghusl for janābah; this is the only narration we have, and we know of no dissent on the point except from Ibn ʿUmar and al-Nakhaʿī, and no one else has agreed with them. The proof is the report of Umm Salamah who said, ‘Messenger of Allah, I am a woman who binds the hair of my head—must I undo it for janābah?’ He answered, ‘No. It is enough for you to pour three double-handfuls of water over your head, then let the water flow over your body, and you will be purified by it.’” Recorded by Muslim.

Al-Mughnī adds: “The four imams are unanimously agreed that undoing the hair is not obligatory.”

They do, however, differ over whether a woman must undo her hair for the ghusl of menstruation.

Imam Aḥmad—according to the well-known view in his school—held that it is obligatory. Mahnā relates: “I asked Aḥmad, ‘Does a woman have to undo her hair because of menstruation?’ He said, ‘Yes.’ I said, ‘Why must she undo it for menstruation but not for janābah?’ He replied, ‘Asmāʾ narrated from the Prophet ﷺ that she is to undo it.’”

The ruling is also supported by the ḥadith in al-Bukhārī (316) and Muslim (1211) in which ʿĀʾishah reports that the Prophet ﷺ said to her: “When you are menstruating, undo your hair and comb it.”

The basic reason for requiring the hair to be undone is to be certain that water reaches what lies beneath it. The concession was granted in the ghusl of janābah because it occurs frequently and undoing the hair would be burdensome, unlike menstruation.

Most scholars, however—including the other three imams—hold that undoing the hair is not required. Their evidence is the Muslim narration in which Umm Salamah said, “Messenger of Allah, I bind the braids of my head—must I undo them for the ghusl of janābah?” He replied, “It is enough for you to pour three double-handfuls of water over your head.”

This is also one report from Imam Aḥmad, preferred by al-Muwaffaq, al-Majd, the commentator, Shaykh Taqī al-Dīn, and others, on the strength of the preceding ḥadith of Umm Salamah.

Shaykh ʿAbd al-ʿAzīz ibn Bāz said: “The sound view is that a woman does not have to undo her hair when performing the ghusl for menstruation, because in some versions of the ḥadith of Umm Salamah recorded by Muslim she said to the Prophet ﷺ:”