Can Women Pray While Wearing Pants?
Question

Can girls pray in pants? What is the clothing that is prescribed in Shari`ah for praying?
Praise be to Allah.What should Muslim women wear to pray?
The clothing that is prescribed for women to pray in is any clothing that covers her entire body apart from the face and hands; it should be loose and opaque, so that it does not show [or form] the shape of any part of her body. 
The fact that a woman’s clothing should cover all of her body while praying is indicated by the Hadith of Umm Salamah (may Allah be pleased with her) who was asked about what clothes a woman should pray in. She said: “She should pray in a Khimar (head cover) and a long chemise that covers the tops of her feet.” (Narrated by Abu Dawud, 639) It was also narrated in a Marfu` report (i.e., attributed to the Prophet (peace and blessings of Allah be upon him)). 
Ibn Hajar (may Allah have mercy on him) said in Bulugh Al-Maram (p. 40): “The Imams classed the Mawquf report as authentic. Ibn Taymiyah (may Allah have mercy on him) said: The well known view is that it is Mawquf, with an Isnad (chain of narration) that stops at Umm Salamah, but it carries the same weight as a Marfu’ report. (Sharh Kitab As-Salah min Al-`Umdah, p. 365)
And the Prophet (peace and blessings of Allah be upon him) said: “Allah does not accept the prayer of a woman who menstruates (i.e., an adult woman) unless her head is covered.” (Narrated by Abu Dawud, 641; At-Tirmidhi, 377; Ibn Majah, 655; classed as authentic by Al-Albani in Sahih Al-Jami`, 7747) 
Should women cover their hands and feet when praying?
It is essential to wear something that covers the entire body apart from the face. The scholars differed as to whether it is obligatory for a woman to cover her hands and feet when praying. 
With regard to the hands, the majority of scholars are of the view that it is not obligatory to cover them. Two views were narrated concerning that from Imam Ahmad (may Allah have mercy on him). Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) was of the view that it is not obligatory. It says in Al-Insaf: “That is the correct view.” 
With regard to the feet, the majority of Malikis, Shafi`is and Hanbalis are of the view that it is obligatory to cover them. This is also the view stated in a Fatwa of the Standing Committee for Issuing Fatwas (6/178): 
Shaykh Ibn Baz (may Allah have mercy on him) said: 
“The entire woman is `Awrah when she is praying, apart from her face. The scholars differed with regard to the hands. Some of them said it is obligatory to cover them, and others said it is allowed to leave them uncovered. The matter is broad in scope, in sha Allah, but it is better to cover them so as to avoid an area of scholarly disagreement. As for the feet, it is obligatory to cover them when praying, according to the majority of scholars.” (Majmu` Fatawa Ibn Baz, 10/410) 
Imam Abu Hanifah, Ath-Thawri and Al-Muzani (may Allah have mercy on them) were of the view that it is permissible for a woman to uncover her feet when praying. This is the view favoured by Shaykh Al-Islam Ibn Taymiyah and by Al-Mardawi in Al-Insaf (may Allah have mercy on them).
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said in Al-Sharh Al-Mumti’ (2/161): 
“There is no clear evidence on this matter. Hence Shaykh Al-Islam Ibn Taymiyah was of the view that a free woman is `Awrah except what appears of her when she is in her home, namely the face, hands and feet. He said: Women at the time of the Prophet (peace and blessings of Allah be upon him) used to wear chemises in their homes, and not every woman had two garments. Hence if menstrual blood got onto her garment, she could wash it and pray in it. The hands and feet are not `Awrah when praying, or with regard to looking. Based on that, there is no solid evidence on this matter, and I follow Shaykh Al-Islam in this regard; this is how we understand it, but we cannot be certain of it, because even if a woman has a garment that reaches to the floor, when she prostrates the bottom of her feet will appear.” (See: Al-Mughni, 1/349; Al-Majmu`, 3/171; Bada’i` Al-Sana’i`, 5/121; Al-Insaf, 1/452; Majmu` Al-Fatawa by Ibn Taymiyah, 22/114) 
But if the garment is thin and shows what is beneath it, and the colour of the skin can be seen, then it is not regarded as covering. (Rawdat At-Talibin by An-Nawawi, 1/284; Al-Mughni, 2/286) 
This is indicated by the Hadith of Abu Hurayrah (may Allah be pleased with him), who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “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…” (Narrated by Muslim, 2128) 
With regard to the words “clothed yet naked”, An-Nawawi (may Allah have mercy on him) said in Al-Majmu` (4/3998): “It was said that they wear thin garments that show the colour of the skin. Ibn `Abd Al-Barr (may Allah have mercy on him) said in At-Tamhid (13/204): What is meant by the phrase “clothed yet naked” is those who wear light garments which show and do not conceal, so they are clothed in name but naked in reality.” 
The evidence that the garment should be loose and opaque is the Hadith of Usamah ibn Zayd (may Allah be pleased with him) who said: the Prophet (peace and blessings of Allah be upon him) gave me a Qubti garment from the gifts that Dihyah Al-Kalbi had given to him, and I gave it to my wife to wear. The Messenger of Allah (peace and blessings of Allah be upon him) said: “Why are you not wearing the Qubti garment?” I said: “I gave it to my wife to wear.” He said: “Tell her to put a Ghalalah underneath it for I fear that it will show the size of her bones.” (Narrated by Al-Bayhaqi in Al-Sunan Al-Kubra, 2/234; classed as sound by Al-Albani in Jilbab Al-Mar’ah Al-Muslimah, p. 131) 
The Qubti garment is a garment made of thin white linen that was made in Egypt. (Lisan Al-`Arab, 7/373)
The Ghalalah is a kind of slip or undergarment.  
Based on this, it is not permissible for a woman when praying to wear tight clothes that show her `Awrah, such as pants or trousers. 
Is praying in pants invalid?
As for the validity of her prayer , even if she goes against this ruling and prays wearing these tight clothes , her prayer is still valid, because what is required is to cover the `Awrah, and she has done that. 
Shaykh Salih Al-Fawzan (may Allah preserve him) said:  
“It is not permissible to wear tight clothes which show the limbs and the shape of a woman’s body. Tight clothes are not permissible for men or women, but it is especially forbidden for women, because the Fitnah [temptation] in their case is greater. 
With regard to praying in particular, if a person prays with his `Awrah covered in such clothes, his prayer is valid in and of itself, because the `Awrah is covered, but he is sinning by praying in tight clothes, because he has transgressed one of the rules of prayer by wearing tight clothes . This is one aspect. Another aspect is that it is a cause of temptation and draws attention, especially in the case of women. So women should cover themselves with loose garments which will cover them and not show any of the limbs of their body or attract attention. So it should not be a thin or see-through garment, rather it should be a concealing garment which will cover the woman completely.”(Al-Muntaqa min Fatawa Ash-Shaykh Salih Al-Fawzan, 3/454)
And Allah knows best.