Musnad Abu Ya'la al-Mawsili 124:4

Abdullah ibn Umar ibn Aban narrated to us, Umar ibn Ubayd al-Tanafisi narrated to us, from Umar ibn al-Muthanna, from Ata al-Khurasani, from Anas ibn Malik, who said: I was with the Messenger of Allah (peace and blessings of Allah be upon him) on a journey. He lagged behind for his need, then he caught up with me and said, "Is there any water?" He performed ablution and wiped over his leather socks (khuffayn), then he caught up with the army and led them in prayer.