Musnad Abu Ya'la al-Mawsili 7:9

Suwayd ibn Sa'id narrated to us, Sharik narrated to us, from 'Asim ibn 'Ubayd Allah, from 'Abd Allah ibn 'Amir ibn Rabi'ah: That 'Umar ibn al-Khattab passed by 'Abd al-Rahman ibn 'Awf while he was circumambulating the House (the Ka'bah), and he was wearing two leather socks (khuffayn) and was chanting (huda'). He said, "By Allah, I do not know which is more amazing: your circumambulation in your two leather socks, or your chanting around the House." He said, "I have done that during the time of one who is better than you: the Messenger of Allah, may Allah bless him and grant him peace."