Reference: Sharh ‘Umdatil-Fiqh – Tape No.9, Question No.17
Question: After having prayed, I noticed a very small bit of impurity on my undergarments, so what should I do? Should I repeat the prayer, or is it sufficient for me to just clean the undergarments?
Response: No; so long as you were unaware [of the impurity on your clothing] until after you had prayed, then you do not need to repeat your prayer. [As for now, then it is sufficient that you] wash your clothes for the next time. As for what has passed, then [in shaa.-Allaah] this is excused, not because you forgot, but because you did not know.