Praise be to Allaah.

It was narrated that Ibn ‘Umar said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Whoever imitates a people is one of them.’” (Narrated by Abu Dawood, al-Libaas, 3512. Al-Albaani said in Saheeh Abi Dawood, (it is) hasan saheeh. No. 3401). 

Al-Munaawi and al-‘Alqami said: i.e., dressing as they dress, following their way of life in clothes and some of the things they do. 

Al-Qaari said: i.e., whoever imitates the kuffaar, such as in how one dresses, etc., or imitates the evil and immoral people, or the Sufis or the righteous, is one of the people whom he imitates, whether they are good or bad.                                                         

Shaykh al-Islam Ibn Taymiyah said in al-Siraat al-Mustaqeem: Imaam Ahmad and others quoted this hadeeth as evidence. This hadeeth at the very least implies that it is haraam to imitate them, as Allaah says (interpretation of the meaning): 

“... And if any amongst you takes them [Jews and Christians] as Awliyaa’ [friends, helpers]), then surely, he is one of them…”

[al-Maa’idah 5:51] 

This is similar to the view of ‘Abd-Allaah ibn ‘Amr who said: “Whoever settles in the land of the mushrikeen and celebrates their Nawroz (new year) and Mahrajaan (festival) and imitates them until he dies will be gathered with them on the Day of Resurrection.” This may be interpreted as referring to absolute imitation which implies kufr and as meaning that imitation in part is therefore haraam; or it may be interpreted as meaning that he is one of them to the extent that he imitates them, whether it is in ideas of kufr, sin or partaking in a ritual. It was narrated from Ibn ‘Umar that the Prophet (peace and blessings of Allaah be upon him) forbade imitating the non-Arabs and said, “Whoever imitates a people is one of them.” This was also mentioned by al-Qaadi Abu Ya’laa. This was quoted by more than one of the scholars to show that it is makrooh to imitate forms of dress of the non-Muslims which are not known among the Muslims. 

See ‘Awn al-Ma’bood Sharh Sunan Abi Dawood. 

Imitating the kuffaar falls into two categories: 

Imitation that is haraam and imitation that is permitted: 

The first type is imitation that is haraam: this means knowingly doing things that are unique characteristics of the religion of the kuffaar and that have not been referred to in our religion. This is haraam and it may be a major sin; in some cases a person may even become a kaafir by doing that, according to the evidence, whether a person does that because he agrees with the kuffaar, or because of his whims and desires, or because of some specious arguments which make him feel that doing it will being him benefit in this world and the next. If it is asked, is the one who does that out of ignorance a sinner, such as one who celebrates Christmas? The answer is that the one who is ignorant is not a sinner because he was unaware, but he has to be told, and if he persists he becomes a sinner. 

The second type is imitation that is permissible. This means doing something which is not originally taken from the kuffaar, but the kuffaar do it too. This does not involve a prohibition on resembling them, but one may miss out on the benefits of differing from them. 

Imitating or resembling the People of the Book (Jews and Christians) and others with regard to worldly matters is permissible only when the following conditions are met: 

1 – That should not be any of their traditions or rituals by which they are distinguished.

2 – That should not be part of their religion. A matter can be proven to be part of their religion though a trustworthy source, such as an aayah of the Qur’aan or a hadeeth of His Messenger, or via well-established reports, such as the prostration of greeting which was permitted to the previous nations.

3 – That should not be anything in Islam which refers specifically to that matter. If there is a specific reference in Islam, either approving or disapproving of it, then we must follow what our religion says about it.

4 – This resemblance should not lead to going against any of the commands of sharee’ah.

5 – That should not involve celebrating any of their festivals.

6 – The resemblance should be only according to what is needed, and no more. 

See al-Sunan wa’l-Athaar fi’l-Nahy ‘an al-Tashabbuh bi’l-Kuffaar by Suhayl Hasan, p. 58-59