Is looking at a Mahram woman's hair or lower leg permissible?

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It is disliked, but not forbidden, for a man to look at the lower leg or chest of his mother, as a means of precaution against inciting desire, according to some views attributed to Ahmad. However, the sound opinion is that looking at areas that usually appear is permissible. The prohibition applies strictly to what is concealed, such as below the navel, because need does not necessitate viewing it, and desire cannot be secured against.