Is washing the skin of the head obligatory during ritual purification?

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Washing the skin of the head is obligatory, regardless of whether the hair is thick or sparse. This obligation extends to everything beneath the hair, such as the skin of the beard and other areas. The evidence for this is the narration from Asma, who asked the Prophet (peace be upon him) about the Ghusl (major ablution) for *janabah* (sexual impurity), and he instructed her to pour water over her head and massage it until the water reached the roots of her hair (*shu'un ra'siha*). Furthermore, there is a narration from Ali (may Allah be pleased with him) that the Prophet (peace be upon him) stated that anyone leaving a single hair dry from *janabah* will be subjected to punishment by the Fire, leading Ali to be meticulous about his hair. This is also established by legal analogy, as the skin beneath the hair is a surface to which water can be delivered without harm, thus making washing it obligatory like the rest of the skin.