Musnad Ahmad 5:607

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Abd al-Malik ibn Maysarah, from al-Nazzal ibn Sabrah, that he witnessed 'Ali pray the Dhuhr prayer, then he sat in the courtyard (al-Rahbah) for the needs of the people. When the time for 'Asr arrived, a vessel (tawr) was brought to him. He took a handful of water and wiped his hands, his forearms, his face, his head, and his feet. Then he drank the remainder while he was standing. Then he said: "Indeed, some people dislike drinking while they are standing, but the Messenger of Allah (peace and blessings of Allah be upon him) did as I have done. And this is the ablution of one who has not broken his state of purity."