Musnad Abu Ya'la al-Mawsili 110:68

Abu Khaythama narrated to us, Sufyan ibn 'Uyaynah narrated to us, from 'Amr, who said: Abu al-Sha'tha' informed me that he heard Ibn 'Abbas hearing the Prophet, may Allah bless him and grant him peace, while he was delivering a sermon: "Whoever does not find a pair of sandals, let him wear leather socks (khuffayn). And whoever does not find an izar (waist wrapper), let him wear trousers (sarawil)."