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The correct view in this is what is proven by the Hadith, and that is the Prophet?s prohibition to a woman in the state of Ihram from covering her face.  So, the woman in Ihram is forbidden to wear a Niqab in any circumstances, whether unrelated men pass by her or not: accordingly, it is unlawful for a woman in a state of Ihram to wear a Niqab, whether she is performing Hajj or ?Umrah. The Niqab is well-known to women:  It is to cover the face with a veil which has two holes for the eyes.  As for the Hadith of ?Aishah, may Allah be pleased with her, it does not contradict the prohibition of wearing a Niqab, because the hadith of ?Aishah, may Allah be pleased with her, it is not mentioned that the women wore the Niqab, it only says that they covered their faces, without using a Niqab.  This is something which is essential if men pass by women, they must cover their faces, because veiling the face from unrelated men is obligatory, so based upon this, we say that wearing the Niqab is unlawful for a woman in the state of Ihram in any circumstances; as for covering her face, It is better for her to uncover her face, it is obligatory for her to cover it, however, she should do so with something other than a Niqab.?  [Fatawa Arkanul-Islam, page 731, question 490]

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It is permissible for the woman who is in a state of Ihraam to cover her face with an adjoining/alternative (cloth) other than the niqaab and the burqa?. [Ikhtiyaaraat al-Fiqhiyyah min Fataawaa Shaikh ul Islaam Ibn Taymiyyah page 117]