Writings on the topic of the veiling and unveiling of women have proliferated in our time. The best that I have seen on this subject is a treatise by my late father Shaykh Mufti Muhammad Shafi’ (Allah have mercy on him) which he called Tafsil al-Khitab fi Tafsir Ayat al-Hijab, which is a section from his Ahkam al-Qur’an (3:393-483), in which he examined the verses and hadiths cited on the subject and exhausted the positions of the fuqaha (jurists) and the statements of the exegetes regarding the limits of hijab and its description. The sum of what he concluded after an extensive study is that the hijab that is legislated and commanded in the Book and the Sunnah has three levels, each above the other in hiddenness and concealment. All of them are mentioned in the Book and the Sunnah and none of them have been abrogated, but they have been prescribed for different circumstances.
In regards to wearing trousers below the ankles (isbal) it is mentioned in a hadith in Sahih al-Bukhari and Sahih Muslim that the Messenger of Allah (Allah bless him and give him peace) said that [on the day of judgment] Allah Most High will not look with mercy towards the person who lowers his izar (lower garment) with pride. The limit of this lowering [of one’s lower garment below the ankles] is mentioned in a second hadith in which the Messenger of Allah (Allah bless him and give him peace) said that [the portion of] the lower garment that is below the ankles is in hellfire. This hadith has been related by al-Bukhari.