If the impurity prevents the validity of the prayer – that is, if it is more than the size of a dirham – his prayer is invalid; because the condition is the purity of the place where the worshipper touches: such as the place of prostration, the standing place, and standing is necessary, which is done with the foot. See: Nafi' al-Mufti p. 225, Jam' al-Rumuz 1: 80, and al-Maraqi p. 208.