From Sahih Bukhari (Hadith 5794):
Narrated Ibn `Umar:A man asked, "O Allah's Apostle What kind of clothes should a Muhrim wear?" The Prophet, said, "A Muhrim should not wear a shirt, trousers a hooded cloak, or Khuffs (socks made from thick fabric or leather) unless he cannot get sandals, in which case he should cut the part (of the Khuff) that covers the ankles
Reference: Bukhari 5794