Musannaf Ibn Abi Shayba 3:4269

Hushaym narrated to us, he said: Yunus informed us, from al-Hakam ibn al-A’raj, who said: I came to Ali ibn Umar while the people were in prayer, and I thought he was not in a state of purity. So I said to him, "O Abu Abd al-Rahman, I have brought you water for purification." He said, "I am in a state of purity, and I have already prayed. So which of the two should I count?" Yunus said: I mentioned this to al-Hasan, and he said, "May Allah have mercy on Abu Abd al-Rahman. He should make the first one the obligatory prayer, and this one a supererogatory (nafilah) prayer."