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 Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) of Allah, that they may remember” [al-A’raaf 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)”
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 Sahaabah (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 sharee’ah.
The Prophet (peace and blessings of Allah be upon him) forbade us to resemble the kuffaar 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 Dawood, 4031; classed as saheeh by al-‘Iraqi in Takhreej Ihya’ ‘Uloom al-Deen (1/342) and by al-Albaani in Irwa’ al-Ghaleel, 5/109.
He also specifically forbade us to resemble them in clothing. It was narrated from ‘Abd-Allah ibn ‘Amr ibn al-‘Aas (may Allah be pleased with him) 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 kuffaar; do not wear them.” Narrated by Muslim, 2077.
Muslim (2069) narrated from ‘Umar that he wrote to the Muslims in Azerbaijan saying: “Beware of luxury and the clothing of the people of shirk.” Narrated by Muslim, 2069.
The garments of the kuffaar which the Muslims are forbidden to wear are those which are worn exclusively by the kuffaar and are not worn by anyone else. As for those which are worn by both kuffaar and Muslims, there is nothing wrong with wearing them and that is not makrooh, because they are not exclusive to the kuffaar.
The scholars of the Standing Committee for Issuing Fatwas were asked about the imitation of the kuffaar that is forbidden. They replied:
What is meant by the imitation of the kuffaar 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 the 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-Tayyibaat [all kinds of Halaal (lawful) things] of food?”
An exception from that is made in the case of that for which there is shar’i evidence that it is forbidden or makrooh, 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 haraam. Similarly, clothing which is worn exclusively by the kuffaar is not permitted for men or for women, because the Prophet (peace and blessings of Allah be upon him) forbade imitating them. And 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 kuffaar, because that is something that is worn by both Muslims and kaafirs 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. But 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.
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 kuffaar 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 doe not cover the chest, forearms, neck, head or face.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) was asked about the definition of imitating the kuffaar. He replied:
The definition of imitation is: doing what is exclusive to those who are being imitated, so imitating the kuffaar 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 is no longer something which distinguishes kaafirs from Muslims, this is not imitation, so it is not haraam on the grounds that it is imitation, unless it is haraam for some other reason.
To sum up our answer:
It is haraam for a Muslim to imitate the kuffaar in things that are exclusive to them, whether it be clothing or anything else, but whatever is not exclusive to the kuffaar, there is nothing wrong with it.
And Allah knows best.
(Source - Islam Q&A)