Musannaf Ibn Abi Shayba 1:1835

Abu Bakr narrated to us, he said: Yazid bin Harun narrated to us, from Hisham, from al-Hasan, regarding the mud of the rain that touches a garment: He said: If he wishes, he may wash it, and if he wishes, he may leave it until it dries, then rub it off.