The Book of Pilgrimage (Kitab Al-Hajj)
Muslim :: Book 7 : Hadith 2650
Ibn 'Abbas (Allah be pleased with both of them) reported: I heard Allah's Messenger (may peace be upon him) say as he was delivering an address: So far as the trousers are concerned, one who does not find lower garment, he may wear them; as also socks, he may wear them who does not find shoes. It concerns the Muhrim.