Ibn Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "When the person in ihram does not find sandals, let him wear leather socks, and let him cut them until they are below the ankles."
Imam al-Shafi'i (may Allah have mercy on him) said: The leather socks should indeed be cut below the ankles, because this is found in the hadith of Ibn Umar (may Allah be pleased with them both), even though it is not found in the hadith of Ibn Abbas (may Allah be pleased with them both), and they are both truthful and precise.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9068