The Book of Hajj-08a

The reciter: Yahya ibn Yahya narrated to us, saying: I read to Malik, from Nafi', from Abdullah ibn 'Umar, that Hafsa (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), said: .

The Shaykh : In this is evidence that the Companions (may Allah be pleased with them), when they saw the Prophet (peace and blessings be upon him) doing something they were not accustomed to, they asked him about it. This is part of the completion of the Shari'ah, for the perfection of the Shari'ah is achieved by the completeness of its branches and its foundations, its details and its general principles. This may come initially in the Book and the Sunnah, or its cause may be a question.
And in his saying: there is evidence that if there is something preventing water from reaching the hair on the head, there is no harm in it. Based on this is what women do today of applying henna to the head, for henna has a substance that prevents water from reaching. But because the purification of the head is light, leniency is granted in this. For this reason, it is permissible to wipe over the turban, and it is permissible to wipe over the head coverings of women according to the well-known view of the school of jurisprudence. But in the case of major ritual impurity, it is necessary that water reaches the roots of the hair.