Wa alaikum salaam

He should repeat the prayers which he believes that this impurity came out after he had performed wudhu. But if the wudhu was correct and nothing came out but the impurity was there without him noticing it, the prayer is valid.

Asalaam alykum, If a person performed Isha salah and five hours later, the person noticed najis that nullify uthu on their underwear, what should that person do repeat Maharib, Isha and sunnah salah because the last time the person made uthu was before maghrib salah.