Musnad Ahmad 27:876

'Abd al-Wahhab narrated to us, from Ayyub, from Nafi', from Ibn 'Umar, who said: A man said: "O Messenger of Allah, what clothes should we wear when we enter into Ihram?" He said: "Do not wear the shirt, nor the trousers, nor the turban, nor the leather socks, unless one does not find sandals, then let him wear them below the ankles. And do not wear the hooded cloak (burnus), nor anything of the clothes that has been touched by wars (a type of dye) or saffron."