Sunan Abi Dawud 125
Another version says:
He performed each part of the ablution three times and washed his feet times without number.
حَدَّثَنَا مَحْمُودُ بْنُ خَالِدٍ، حَدَّثَنَا الْوَلِيدُ، بِهَذَا الإِسْنَادِ قَالَ فَتَوَضَّأَ ثَلاَثًا ثَلاَثًا وَغَسَلَ رِجْلَيْهِ بِغَيْرِ عَدَدٍ ‏.‏