What is the ruling regarding the 'Isha' prayer if yellow spots, presumed to be madhy (pre-seminal fluid), were found on one's clothes, and the clothes were changed before the prayer, while being ignorant of the time when this madhy occurred?

If someone prays while knowing that there is impurity on their garment, body, or place, they must repeat the prayer. However, if they become aware of it after the prayer, then, according to the preponderant opinion, they do not need to repeat it. If one doubts the presence of impurity before the prayer, they are not obligated to repeat it, because the default assumption is the prayer's validity. Therefore, the questioner's prayers before knowing about the madhy (pre-seminal fluid) are valid, and he does not need to repeat them.