Sayyiduna Aws bin Abi Aws (may Allah be pleased with him) narrates from his father, who said: We passed by one of the watering places of the Bedouins, and my father stood up, urinated, then performed ablution and wiped over his sandals. I asked: Will you not take them off? He said: "I do not go beyond what I saw the Messenger of Allah (peace and blessings be upon him) doing."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 602