Shaykh Muhammad ibn Saalih al-Uthaymeen rahimahullaah was also asked:
Q: As you no doubt know, the place of trial (fitnah) in a woman is concentrated in her inner body (that she covers) – so if it (her inner body) becomes apparent, trial (fitnah) and evil become widespread. So what is permissible for a woman to expose from her body, and what is the ruling of a woman looking at another woman’s awrah (parts that must be covered)?
A: It is obligatory upon the woman to wear clothing that is in accordance with the legislation, clothing that provides covering; and the clothing of the female companions as Shaykhul-Islaam ibn Taymiyyah and other than him have mentioned was from their hands to their ankles in their houses, meaning: from the palm of their hands to the ankle of the leg.
So if they went out they would wear long clothes, (the length of which) exceeded beyond their feet by a handspan. And the Prophet sallAllaahu `alaihi wa sallam allowed them (to extend their clothes) to a forearms length so that their feet would be covered. This is as it relates to the covered woman, so if she were to lift up her clothes then she is from the women that are dressed but appear to be naked.
As for the woman who looks (at another woman) then it is not permissible for her to look at the awrah (parts that must be covered) of another woman. Meaning: it is not permissible for her to look at that which is between the navel and the knee; such as if a woman was answering the call of nature for example – then it is not allowed for the (another) woman to look at her, because she would be looking at her awrah (the parts of her that must be covered).
As for that which is above the navel or below the knee then if a woman were to uncover that for a need; such as if she were to lift her clothes and uncover her shin when walking by clay-like ground, or if she wanted to wash her shin and another woman was present – then there is no harm in that. Or if she was to reveal her breast to breastfeed her child in the presence of other women then there is no harm in that.
However it is not to be understood that which some of the ignorant women understand: that it means a woman can wear clothes which only cover the region between her navel and her knee. This is an error, a grave error regarding the Book of Allaah, and the Sunnah of the Messenger of Allaah sallAllaahu `alaihi wa sallam, and regarding the Legislation of Allaah, and the pious predecessors of this nation. Whoever says: The woman can just wear trousers that cover that which is between the navel and the knee; and this is from the clothing of the Muslim women?! This is impossible.
So it is obligatory upon the woman to wear outer-garments (that cover) from the hand to the ankle. As for a woman who looks at another woman, then she can see her chest and her calf, however she cannot look at that which is between the navel and knee if a woman was to uncover her clothes. So another woman does not look at what is between the navel and knee.
[Source: The Shaykh’s official website: http://www.ibnothaimeen.com/all/books/article_16956.shtml]