Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that he heard the Messenger of Allah (peace and blessings be upon him) delivering a sermon, saying: "When the person in ihram (muhrim) does not find sandals, let him wear leather socks (khuffs), and when he does not find a waist-wrap (izar), let him wear trousers."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9066