If I find mathy on my underwear after I finish the prayer, then must I repeat the prayer or prayers since the last time I was certain that my clothes were pure? I do not know when it came out as I did not feel it.
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
According to the most correct view of the scholars, you do not have to repeat the prayer if you find impurities on your clothes that you did not know of, whether this impurity is Mathi or otherwise.
However, if you have discharged Mathi and you did not perform ablution and you did not know about its discharge except until after the prayer, then you have to repeat the prayer that you had prayed without ablution.
If you are not sure of the time the impurity was discharged, then you attribute it to the nearest time that is probable for its discharge. For example, if you doubt whether the discharge of Mathi was before or after the prayer, then in principle it had occurred after the prayer, and consequently you do not have to repeat it.
Allah knows best.
You can search for fatwa through many choices