I say, and with God's success: The act of cleaning oneself after relieving oneself (istinja) is obligatory if the impurity is more than the size of a dirham. If there is impurity greater than the size of a dirham and one prays with it, their prayer is invalid. However, if the impurity is the size of a dirham or less, their prayer is valid, and it is excused; because one who cleansed themselves with a stone instead of water, their prayer is valid by consensus, and the stone does not completely remove the impurity. Therefore, if one sits in a small amount of water, it becomes impure, indicating that it is excused and measured by the dirham. See: Al-Tawdeeh Sharh Muqaddimah Abi Al-Layth 94/b, Badai' Al-Sanai' 1/18, and Al-Ikhtiyar 1/48, and God knows best.