Forbidden Types of Disbelievers’ Clothing
Question

May Allah reward you for your efforts. In the time of our Prophet Muhammad (peace be upon him), how did the Muslims distinguish themselves from the disbelievers in their dressing? Did the disbelievers of Mecca also wear long robes (commonly known today as kurtas, jallabias etc.)? So can the wearing of the loose robe be considered as an Islamic dress code?
Praise be to Allah.Islamic dress code
Clothing is one of the blessings that Allah has bestowed upon His slaves. It covers the `Awrah and protects against heat and cold. Allah has bestowed this blessing upon them as He says (interpretation of the meaning): 
“O Children of Adam! We have bestowed raiment upon you to cover yourselves (screen your private parts) and as an adornment; and the raiment of righteousness, that is better. Such are among the Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) of Allah, that they may remember.” [Al-A`raf 7:26] 
“… and has made for you garments to protect you from the heat (and cold), and coats of mail to protect you from your (mutual) violence. Thus does He perfect His Favour unto you, that you may submit yourselves to His Will (in Islam).” [An-Nahl 16:81]
The basic principle concerning garments is that they are permissible, and the Muslim may wear whatever he wants of things that he has made or that Muslims or others have made for him. This is what the Companions (may Allah be pleased with them) did in Makkah and elsewhere. Those who became Muslim did not wear special garments that were just for them. 
The Prophet (peace and blessings of Allah be upon him) wore Syrian garments and Yemeni cloaks, and the people who made them were not Muslims. What matters is that the garment should meet the conditions stipulated by Shari`ah . 
Prohibition to imitate the disbelievers
The Prophet (peace and blessings of Allah be upon him) forbade resembling the disbelievers in general – in clothing and in other ways. The Prophet (peace and blessings of Allah be upon him) said: “Whoever imitates a people is one of them .” (Narrated by Abu Dawud, 4031; classed as authentic by Al-`Iraqi in Takhrij Ihya’ `Ulum Ad-Din (1/342) and by Al-Albani in Irwa’ Al-Ghalil, 5/109.) 
What clothes are prohibited in Islam?
The Prophet also specifically forbade resembling them in clothing. `Abdullah ibn `Amr ibn Al-`As (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) saw him wearing two garments dyed with safflower and he said to him: “These are the garments of the disbelievers; do not wear them.” (Narrated by Muslim, 2077) 
Muslim (2069) narrated from `Umar (may Allah be pleased with him) that he wrote to the Muslims in Azerbaijan saying: “Beware of luxury and the clothing of the people of shirk.” 
The garments of the disbelievers which the Muslims are forbidden to wear are those which are worn exclusively by the disbelievers and are not worn by anyone else. As for those which are worn by both disbelievers and Muslims, there is nothing wrong with wearing them and that is not disliked, because they are not exclusive to the disbelievers. 
The scholars of the Standing Committee for Issuing Fatwas were asked about the imitation of the disbelievers that is forbidden. They replied: 
“What is meant by the imitation of the disbelievers that is forbidden is resembling them in the customs that are exclusive to them, or in the religious beliefs and acts of worship that they have invented, such as imitating them by shaving the beard. 
With regard to wearing pants , suits and so on, the basic principle is that all types of clothing are permissible, because this is a matter of customs. Allah says (interpretation of the meaning): 
“Say (O Muhammad): Who has forbidden the adornment with clothes given by Allah, which He has produced for His slaves, and At-Tayyibat [all kinds of lawful things] of food?” [Al-A`raf 7:32]
An exception from that is made in the case of that for which there is Shar`i evidence proving something to be forbidden or disliked, such as silk for men, or that which shows the `Awrah because it is thin and shows the colour of the skin beneath it, or because it is tight and shows the shape of the `Awrah , because in that case it is like showing it, and showing it is prohibited. 
Similarly, clothing which is worn exclusively by the disbelievers is not permitted for men or for women, because the Prophet (peace and blessings of Allah be upon him) forbade imitating them. Also, it is not permitted for men to wear women's clothing or women to wear men’s clothing, because the Prophet (peace and blessings of Allah be upon him) forbade men to imitate women and women to imitate men. 
Wearing pants or trousers is not something that is exclusive to the disbelievers, because that is something that is worn by both Muslims and disbelievers in many countries. Rather some people do not like to wear such things in some countries because they are not used to it and it is contrary to the customary dress of the people, even though it is customary among other Muslims.  However, if the Muslim is in a country where the people are not accustomed to such clothing, it is better for him not to wear it when praying or in public gatherings or in the street.” (Fatawa Al-Lajnah Al-Da’imah, 3/307-309) 
They also said:
“Muslim men and women should be keen to adhere to Islamic customs and attitudes, and to follow the Islamic way in their expressions of joy and sorrow and in their clothing, food, drink and all their affairs. 
It is not permissible for them to imitate the disbelievers in their clothing by wearing tight clothes that show the shape of the `Awrah, or by wearing thin and see-through clothes that show the `Awrah and do not conceal it, or by wearing short clothes that do not cover the chest, forearms, neck, head or face.” (Fatawa Al-Lajnah Ad-Da’imah, 3/306, 307) 
Definition of imitating the disbelievers
Shaykh Muhammad ibn Salih Al-`Uthaymin (may Allah have mercy on him) was asked about the definition of imitating the disbelievers. He replied: 
“The definition of imitation is: doing what is exclusive to those who are being imitated, so imitating the disbelievers refers to a Muslim doing something that is distinct and exclusive to them. 
With regard to things that have become widespread among the Muslims and are no longer something which distinguishes disbelievers from Muslims, then this is not imitation, so it is not prohibited on the grounds that it is imitation, unless it is prohibited for some other reason. 
What we have said is what is indicated by this word, and it was explained in a similar way by the author of Fath Al-Bari when he said (10/272): Some of the Salaf regarded it as disliked to wear the burnous because it was the garment of monks. Malik (may Allah have mercy on him) was asked about it and he said: ‘There is nothing wrong with it. It was said to him: But it is the clothing of the Christians. He said: It is worn here.’ End quote. 
I say: It would have been stronger evidence if, when Malik (may Allah have mercy on him) was asked whether wearing the burnous is prohibited, he had quoted as evidence the words of the Prophet (peace and blessings of Allah be upon him): “(The pilgrim in Ihram) should not wear a shirt, pants, a burnous…”  
In Al-Fath (10/307) it also says: If we say that they (silken saddlepads) are forbidden because that is an imitation of the non-Arabs, then this is a religious reason, but that was their symbol at that time when they were disbelievers, but now it is not something it is unique to them so this meaning no longer applies, so it is no longer disliked. And Allah knows best.” (Majmu` Fatawa Ash-Shaykh Ibn `Uthaymin, 12/290) 
Shaykh Salih Al-Fawzan (may Allah preserve him) said: 
“It is permissible to wear the garments of disbelievers so long as they are not known to be impure, because the basic principle is that things are pure, and that is not altered by doubt. So what they have woven and dyed is permissible, because the Prophet (peace and blessings of Allah be upon him) and his companions used to wear things that had been woven and died by the disbelievers.” (Al-Mulakhkhas Al-Fiqhi, 1/20) 
To sum up, it is prohibited for a Muslim to imitate the disbelievers in things that are exclusive to them, whether it be clothing or anything else, but whatever is not exclusive to the disbelievers, there is nothing wrong with it.
And Allah knows best.