Al-Matalib al-'Aliyah 172 — The description of ghusl

It is narrated from Na'im (may Allah have mercy on him), the freed slave of Sayyidah Umm Salamah (may Allah be pleased with her): I asked the Mother of the Believers Sayyidah Umm Salamah (may Allah be pleased with her) about a man's ghusl, and she said: "He should clean his hair well and drench his skin thoroughly with water." And I asked her about a woman's ghusl, and she said: "She should clean the skin of her scalp well, but she does not need to undo her (braided) hair."
This report describes the manner of the bath for janabah: in ghusl it is necessary to make water reach the roots of the hair and the whole skin; however, if a woman has plaited her hair into braids, it is sufficient for her merely to make water reach the skin of the scalp, and undoing the hair is not obligatory.