Musnad Ahmad 25:177

Yahya narrated to us, from Ibn Jurayj, who said: 'Amr ibn Dinar informed me that Abu al-Sha'tha' informed him that Ibn Abbas informed him: That he heard the Messenger of Allah, peace and blessings of Allah be upon him, delivering a sermon, and he was saying: "Whoever does not find an izar (waist-wrapper) but finds trousers, let him wear them. And whoever does not find a pair of sandals but finds a pair of khuffs (leather socks), let him wear them." I said: "Did he not say 'let him cut them'?" He said: "No."