Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: When this verse was revealed: "In it are men who love to purify themselves" [Al-Tawbah: 108], the Prophet (peace and blessings be upon him) sent (someone) to Uwaym bin Sa'idah (may Allah be pleased with him) and asked: "What is this purification for which Allah, the Mighty and Majestic, has praised you?" They said: O Messenger of Allah! Not a man or woman among us comes out from relieving themselves without washing their private part - or he said: their backside. So the Prophet (peace and blessings be upon him) said: "This is it (that purification)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11019