Musnad Ahmad 25:1256

Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, and Rawh said: Ibn Jurayj narrated to us, he said: ‘Amr ibn Dinar informed me that Abu al-Sha’tha’ informed him, he said: Ibn ‘Abbas narrated to me that he heard the Messenger of Allah, may Allah bless him and grant him peace, while he was delivering a sermon, 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."