Musnad Ahmad 10:13

Hashim ibn al-Qasim narrated to us, Sharik narrated to us, from Asim ibn Ubayd Allah, from Abdullah ibn Amir ibn Rabiah, who said: Umar ibn al-Khattab, may Allah be pleased with him, heard the voice of Ibn al-Mughtarif—or Ibn al-Gharaf the camel driver—in the middle of the night while we were traveling to Makkah. Umar urged his mount forward until he entered among the people, and behold, he was with Abd al-Rahman. When dawn broke, Umar said: "Prepare now, be silent now, the dawn has broken, remember Allah." He said: Then he noticed leather socks (khuffayn) on Abd al-Rahman. He said: "And leather socks?" He replied: "I have worn them with someone who is better than you—or with the Messenger of Allah, peace and blessings of Allah be upon him." Umar said: "I adjure you to take them off, for I fear that people will look at you and follow your example."