(a) Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: When this verse, 'In it are men who love to purify themselves' [Surah al-Tawbah: 108], was revealed, the Prophet (peace and blessings be upon him) sent for Uwaym bin Sa'idah and asked: "What is this purification for which Allah has praised you?" He said: O Messenger of Allah, whichever man or woman among us goes out to relieve themselves washes their private part with water. The Prophet (peace and blessings be upon him) said: "It is about this that the verse was revealed."
(b) It is narrated from Sayyiduna Hudhayfah bin al-Yaman (may Allah be pleased with him) that when he urinated he would clean himself with water, and from Sayyidah Aishah (may Allah be pleased with her) that it is Sunnah for a woman to wash her front private part.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 512