Question
If the worshiper places his knees on a place of impurity, does his prayer become invalid?
Answer
His prayer is valid; because the condition is the purity of the place where the worshipper touches: such as the place of prostration and the standing place. As for the parts that touch the ground as a Sunnah: such as the hands and knees, their purity is not a condition. See: Fath Bab al-Ianaya 1: 220.