I say, and with God's success: If the impurity is severe, like urine and blood, then the amount that is excused is that of a dirham or less. If it is mild, like the urine of animals whose meat is permissible to eat, such as sheep, then the amount that is excused is that of a quarter of a garment or less. It is permissible to pray with it; otherwise, it is not permissible. If the impurity is more than that, the prayer is invalid, and the impurity must be removed. And God knows best.