This is proven from the three major sources of Islamic law:
1. The Qur’an. Says Allah in the Qur’an, “O you who believe, when you rise for prayer, wash your faces and your hands up to the elbows and lightly rub your heads and (wash) your feet up to the ankles (al-Mai’dah 6).
2. The Sunnah. Abu Hurairah reported that the Messenger of Allah said, “Allah does not accept the prayer of one who nullified his ablution until he performs it again.” (Related by al-Bukhari, Muslim, Abu Dawud and at-Tirmizhi.)
3. The Consensus. There is a consensus of scholarly opinion that ablution is part of Islamic law. Therefore, it is a recognized fact of the religion.