Musnad Ahmad 27:91

Sufyan narrated to us, from al-Zuhri, from Salim, from his father, who said: A man asked the Messenger of Allah, peace and blessings of Allah be upon him: "What garments should the *muhrim* (pilgrim in a state of ihram) wear?" And Sufyan said once: "What garments should the *muhrim* leave off?" He said: "He shall not wear the shirt, nor the hooded cloak (*burnus*), nor the trousers, nor the turban, nor a garment touched by *wars* (a yellow dye), nor saffron, nor the leather socks (*khuffayn*), except for one who does not find sandals. So whoever does not find sandals, let him wear the leather socks, and let him cut them so that they are below the ankles."