Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.015B.

Shoes may be purified by rubbing them against the ground, as long as the remains of the impurity are removed. Abu Hurairah narrated that the Messenger of Allah, upon whom be peace, said, “If one of you stepped in some filth, the dirt will purify his shoes.” Related by Abu Dawud. In another narration it states, “If one of you steps in some filth with his shoes on, the dirt will purify them.” Abu Sa’eed reported the Prophet, upon whom be peace, saying, “When a person comes to the mosque, he should look at his shoes. If he finds any filth on them, he should wipe them against the ground and pray in them.” (Related by Ahmad and Abu Dawud.) Since shoes are repeatedly exposed to filth, it is sufficient just to wipe them against the ground. This is similar to the case of defecation. In fact, it is stronger than that case, as defecation usually occurs only two or three times a day.