The interpretation of this narration is that we carry it upon the one who entered his prayer with tayammum and then experienced a hadath forgetfully before the two testimonies; in that case, he should perform ablution if he has found water and complete the prayer with the two testimonies, and it is not obligatory upon him to repeat it, just as it is obligatory upon him to complete it if he experiences a hadath before that, as we have explained in the Book of Purity.
Commentary #30