Allaah has forbidden the Muslim to imitate the disbelievers, and the Prophet (blessings and peace of Allaah be upon him) spoke very sternly concerning that, as he said: “Whoever imitates a people is one of them.” Narrated by Abu Dawood (4031) and classed as saheeh by al-Albaani inSaheeh Sunan Abi Dawood.
The prohibition on imitating the kuffaar applies only to that which is exclusively theirs and which is not common to them and the Muslims.
What explains the meaning of exclusivity is that if the person who does that action is seen it would be said of him that he belongs to the group which we are forbidden to imitate. This can only apply to actions which are not done by anyone but that group. As for actions which are common to them and the Muslims, it is not correct to say that doing this is regarded as the forbidden type of imitation, because this action is not exclusive to them.
Based on that, the rulings on things that are only forbidden because they are imitation of the mushrikeen vary according to time and place, and according to different traditions and customs.
If that kind of clothing in a particular country is worn only by the kuffaar, then it is haraam for the Muslim to wear it in that country, but if in another country it is worn by both Muslims and kaafirs, then it is permissible to wear it in that country. Nowadays, wearing pants or suits is not exclusive to the kaafirs; rather they are worn by Muslims in most countries and they do not think that wearing it is imitation of the kaafirs, because it is not exclusively theirs.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: What is the guideline on the issue of imitating the kuffaar?
Imitation of the kuffaar may be in appearance, clothing, food and other things because it is a general word which refers to a person doing something that is done exclusively by the kuffaar, in such a way that whoever sees him would think that he is one of the kuffaar. This is the guideline. But if the thing has become widespread among both Muslims and kaafirs, then this imitation is permissible, even if it was originally taken from the kuffaar, so long as it is not haraam in and of itself, such as wearing silk. End quote.
He was also asked: what is the definition of imitating the kuffaar?
The definition of imitating (the kuffaar) is when the imitator does something that is exclusive to the one being imitated. So imitation of the kuffaar means that a Muslim does something that is exclusively theirs. But with regard to that which has become widespread among the Muslims and is not a distinguishing characteristic of the kuffaar, this is not imitation, and it is not haraam on the grounds of imitation, unless it is haraam for some other reason. What we have said is the implication of these words. Something similar was stated by the author of al-Fath [Ibn Hajr], when he said (10/272): Some of the salaf regarded it as makrooh to wear the burnoose, because it was the clothing of monks. Maalik was asked about that and he said: There is nothing wrong with it. It was said: But it is the clothing of the Christians. He said: It was worn here.
Ibn [Ibn ‘Uthaymeen] said: If Maalik hadquoted as evidence the words of the Prophet (blessings and peace of Allaah be upon him) when he was asked what should the pilgrim in ihram wear? and he said: “He should not wear a shirt or a turban or pants or a burnoose…”, that would have been better.
In al-Fath (1/307) it also says: If we say that it is forbidden (i.e., purple saddle pads) because it is imitation of the non-Arabs, that is for a religious reason. But that was one of their unique characteristics at that time, when they were kuffaar. But now that it is no longer one of their unique characteristics, this meaning no longer applies, so it is no longer makrooh.
And Allaah knows best.