Emulation of the non-Arabs in their dress
from the “M‘iyar al M‘urab” of Al Wansharisi (died 914), V.11/Pp.27-28
The pious jurist, the Mufti, Abu Abdullah Al Mawwaq was asked about the ‘Darnadin’ is it permissible to wear? It being a piece of roman clothing; emulation not being applicable when compared to the benefits wearing it has; it is a useful, economical piece of clothing which protects one from the cold.
The rationale here is that the Shariah does not forbid those things related to emulation while at the same time permitting them; The Messenger of Allah built the trench on the border of Medinah, emulating the Non-Arabs [in this action], astounding the Confederate forces, then later they came to know that this was done at the direction of Salman Al Farisi .
This [piece of clothing known as the] ‘Darnadin’ mentioned is economical, it is not extravagant, and is very useful, protecting one against the cold; It has been authentically narrated from the Messenger of Allah that he had worn a Roman coat with narrow sleeves .
Looking at this issue in another light, this have been stated explicitly by the esteemed scholars; the Messenger of Allah forbade the Arabs from emulating the Non-Arabs  yet it is not related from anyone that when the Non-Arab Delegations came to him that he forbade them from wearing their own clothing and ordered them to dress like the arabs…. (emphasis added).
 See Al Taj wa _l Iklil of Al ‘Abdari, V.1/P.502
 See Al Fusul fi Sirat _l Rasul of Ibn Kathir, P.163
 Narrated by Al Tirmidhi (1768), Al Nisa_i (125), Ibn Majah (3563) and Ahmad (18265).
 Narrated by Ahmad (301) as the statement of Omar, who then narrates that the Messenger of Allah forbade the wearing of silk for men. The context of the narration shows that Omar understood this prohibition to be one forbidding emulation of the non-Arabs of that time in their extravagance. Similar to this is his prohibition of sitting on silk carpets (see Ibn Majah 3654) and tiger skins (see Ibn Majah 3656).