What is the ruling on the prayer of one who remembers the impurity of his clothes while he is in the third unit (rak'ah) of the 'Asr prayer?

Whoever remembers the presence of impurity on his clothes or body during prayer, it is obligatory for him to exit the prayer to remove it, whether he is an imam or a follower. It is not permissible to continue the prayer while aware of the impurity. If he does so, his prayer is invalid, and he is sinful.