Musnad Ahmed: Hadith 1668

"Umar bin al-Khattab (رضي الله عنه) heard the voice of Ibn al-Mughtarif - or Ibn al-Gharif - the camel driver [one who urges camels on with his singing or chanting] in the middle of the night, when we were setting out for Makkah. "Umar made his camel speed up until he joined the people, and found himself with "Abdur-Rahman. When dawn broke, "Umar said: Now keep quiet, for dawn has broken; remember Allah. Then he saw ‘Abdur-Rahman wearing khuffain (leather slippers) and he said: Leather slippers?! He said: I wore them with someone whom was better than you, or with the Messenger of Allah (ﷺ). "Umar said: I urge you to take them off, for I am afraid that the people will see you and follow your example.