It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "When the muhrim (one in ihram) does not find a waist-wrap (izar), let him wear trousers, and whoever does not find sandals, let him wear leather socks (khuffs)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12354