Forbidden Types of Imitating the Disbelievers
Question

I heard some people say that wearing pants and suits is prohibited because it is an imitation of the disbelievers. Is this true?
Praise be to Allah.The Prohibition of Imitating Disbelievers
Allah has forbidden the Muslim to imitate the disbelievers , and the Prophet (blessings and peace of Allah be upon him) spoke very sternly concerning that, as he said: “Whoever imitates a people is one of them.” (Narrated by Abu Dawud, 4031; classed as authentic by Al-Albani in Sahih Sunan Abu Dawud)
Guidelines concerning imitation of the disbeliever
The prohibition on imitating the disbelievers 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. 
Is wearing pants part of imitating the disbelievers?
Based on that, the rulings on things that are only forbidden because they are imitation of the disbelievers 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 disbelievers, then it is prohibited for the Muslim to wear it in that country, but if in another country it is worn by both Muslims and disbelievers, then it is permissible to wear it in that country. Nowadays, wearing pants or suits is not exclusive to the disbelievers ; rather they are worn by Muslims in most countries and they do not think that wearing it is imitation of the disbelievers, because it is not exclusively theirs. 
Based on this, wearing it is permissible and there is nothing wrong with it. 
We have previously quoted a Fatwa of the Standing Committee for Issuing Fatwas which says that it is permissible to wear pants and suits , and it is not imitation of the disbelievers. 
Shaykh Muhammad ibn `Uthaymin (may Allah have mercy on him) was asked: What is the guideline on the issue of imitating the disbelievers? 
He replied: 
Imitation of the disbelievers 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 disbelievers, in such a way that whoever sees him would think that he is one of the disbelievers. This is the guideline. But if the thing has become widespread among both Muslims and disbelievers, then this imitation is permissible, even if it was originally taken from the disbelievers, so long as it is not prohibited in and of itself, such as wearing silk.” (Majmu` Durus wa Fatawa Al-Haram Al-Makki, 3/367) 
Definition of imitating the disbelievers
Ibn `Uthaymin (may Allah have mercy on him) was also asked: what is the definition of imitating the disbelievers? 
He replied: 
“The definition of imitating (the disbelievers) is when the imitator does something that is exclusive to the one being imitated. So imitation of the disbelievers 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 disbelievers, this is not imitation, and it is not prohibited on the grounds of imitation, unless it is prohibited 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 disliked to wear the burnoose, because it was the clothing of monks. Malik 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.” 
`Uthaymin  ibn `Uthaymin (may Allah have mercy on him) said: “If Malik had quoted as evidence the words of the Prophet (blessings and peace of Allah 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 disbelievers. But now that it is no longer one of their unique characteristics, this meaning no longer applies, so it is no longer disliked. And Allah knows best." (Fatawa Al-`Aqidah,   245) 
The purple saddle pad was a kind of thick blanket that the rider put beneath him on the horse. 
And Allah knows best.