It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to the people of Quba: "What is this purification for which you have been singled out in this verse: 'In it are men who love to purify themselves, and Allah loves those who purify themselves' [Surah al-Tawbah: 108]?" They said: "O Messenger of Allah, every one of us who relieves himself washes his rear (private part)."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7524