Praise be to Allah.
The clothing in which a woman appears before non-mahram men must meet eight conditions:
1- It should cover the entire body, including the face and hands. The evidence for that has been discussed in the answer to question no. 11774.
2- It must be wide and loose, not showing the size of her limbs or the shape of her body.
3- It should not be thin, showing the colour of her skin.
4- It should not be an adornment in and of itself, such as clothing decorated with embroidery.
5- It should not be perfumed.
6- It should not resemble the clothing of men.
7- It should not resemble the dress of kaafir women.
8- It should not be a dress of fame and vanity.
See Adaab al-Zafaaf by Shaykh al-Albaani (may Allaah have mercy on him), p. 177; Hijaab al-Mar’ah al-Muslimah, p. 16-111; ‘Awdat al-Hijaab, 3/145-163.
Based on this, a woman should not appear in front of men wearing pants or trousers because they show the shape of the woman’s legs
2 – Wearing them is an imitation of men
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: I think that the Muslims should not follow these fashions and kinds of clothes that come to us from here and there. Many of them are not in accordance with proper Islamic dress which involves covering the woman completely, like the short or very tight or thin clothes. That includes pants, because they show the size of a woman’s legs, as well as her stomach, waist, breasts, etc. Wearing it comes under the description given in the saheeh hadeeth: “there are two types of the people of Hell whom I have not seen: people with whips like the tails of cattle, with which they strike the people, and women who are clothed yet naked, walking with an enticing gait, with something on their heads that looks like the humps of camels, leaning to one side. They will never enter Paradise or even smell its fragrance, although its fragrance can be detected from such and such a distance.”
This hadeeth was narrated by Muslim, 2128.
End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen.
With regard to wearing pants under a jilbaab, there is nothing wrong with that, rather it is more covering and more protective, if the jilbaab is wide and concealing and does not have any splits that show what is underneath it.
Shaykh ‘Abd al-Razzaaq ‘Afeefi (may Allaah have mercy on him) said: If a woman wears pants and puts over them a wide garment then she will not be resembling men, so long as she wears them underneath her garments. End quote from Fataawa al-Shaykh ‘Abd al-Razzaaq ‘Afeefi, p. 573.
The basic principle is that the woman’s jilbab should be wide and cover the tops of her feet, because of the report narrated by al-Tirmidhi (1731), al-Nasaa’i (5336), Abu Dawood (4117) and Ibn Maajah (3580) from Ibn ‘Umar (may Allaah be pleased with him) who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever lets his garment drag out of pride, Allaah will not look at him on the Day of Resurrection.” Umm Salamah said: “What should women do with their hems?” He said: “Let them down by a handspan.” She said: “But then their feet will be uncovered.” He said: “Then a cubit, but no more than that.” This hadeeth was classed as saheeh by al-Albaani in Saheeh Sunan al-Tirmidhi.
Al-Baaji said: The words of Umm Salamah (may Allaah be pleased with her) regarding lengthening the hem by a handspan – “But then their feet will be uncovered” – show that this length was not sufficient to cover them, because the movement of her feet when walking quickly, combined with the shortness of the hem, would lead to them being uncovered. When she pointed that out to the Prophet (peace and blessings of Allaah be upon him), he said: “Then a cubit, but no more than that.” End quote from al-Muntaqa.
Shaykh Saalih al-Fawzaan (may Allaah preserve him) was asked: Is it mustahabb or waajib for a woman to make her dress long? Is wearing socks on the feet sufficient if the dress is shortened only so much that nothing of the calf shows? How should a woman lengthen her dress by a cubit – below the ankle or below the knee?
He replied: What is required of the Muslim woman is to cover all her body in front of men, hence she is allowed to lengthen her garment by a cubit so as to cover her feet, whereas men are forbidden to let their garments hang below their ankles. This indicates that women are required to cover all of their bodies. If a woman wears socks that is doing something extra to be more on the safe side with regard to covering, which is something good. She should also make her garment long, as described in the hadeeth. And Allaah is the Source of strength. End quote. Al-Muntaqa min Fataawa al-Shaykh al-Fawzaan, 5/334.
The point is that a woman’s jilbab should be wide and come down to her ankles or more. If it is short and comes a handspan above the ankles, that is not permissible even if she covers her calves and feet with pants or socks, because that is imitating men who are enjoined to make their garments short, above the ankles. It also shows the shape and size of her feet.
And Allah knows best.