I say, and with God's help: If the impurity is severe, like urine and blood, then the excusable amount is the size of a dirham or less. If it is lighter, like the urine of animals whose meat is eaten, such as sheep, then the excusable amount is the size of a quarter of a garment or less. It is permissible to pray with it; otherwise, it is not permissible. And God knows best.