Musannaf Ibn Abi Shaybah

Hazrat Hakam bin A'raj says that I went to Hazrat Ibn Umar (may Allah be pleased with them). At that time, people were performing the Zuhr prayer. I thought that he did not have ablution. I said to him, "O Abu Abdur Rahman! Shall I bring water for you to perform ablution?" He said, "I have ablution and I have already performed the prayer. Which of the two should I count?" Yunus says that I mentioned this matter to Hasan, so he said, "May Allah have mercy on Abu Abdur Rahman! He considered the first prayer he performed as obligatory and the other as voluntary."