If the henna paste was applied (on the hair of the head) in such a manner, that not even a quarter part of the hairs (on the head) remained untouched, and the henna is so layered, that the effect of the moisture (by masah) would not reach the hairs, then so to do masah on top of such layered henna paste is not correct.

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للمفتي محمد أشرف الاشاعتي

Vol.1 Pg.95