مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2514
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
Jabir bin Zaid narrated that he heard 'Abdullah bin ‘Abbas say that he heard the Messenger of Allah (ﷺ) deliver the khutbah in ‘Arafah, [saying]: "Whoever does not have sandals, let him wear khuffain (leather slippers) and whoever does not have an Izar, let him wear pants."
Musnad Ahmad 2526lts isnad is saheeh, al-Bukhari (1841) and Muslim (1178)]