Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.019B.

To do so, he can use a rock, stone or any other pure matter. One may use only water to clean the area, or any combinations of purifying agents. ‘Aishah reported that the Messenger of Allah, upon whom be peace, said, “When one of you goes to relieve himself, he should clean himself with three stones.” (Related by Ahmad, anNasa’i, Abu Dawud and ad-Daraqutni).

Anas related that the Messenger of Allah would enter the privy, and that Anas and another boy would carry the water container and spear for him. The Prophet would clean himself with water.” (Related by al-Bukhari and Muslim.)

Ibn ‘Abbas related that the Messenger of Allah, upon whom be peace, passed by two graves and said, “They are being punished. But they are not being punished for a great matter (on their part). One of them did not clean himself from urine and the other used to spread slander.” (Related by “the group.”)

Anas also related the Prophet as saying, “Purify yourselves from urine, as most punishment in the grave is due to it.”