Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.051.
Section : Sperm on the clothes during prayer.
If a man does not know how the sperm got on his clothes, and he has already prayed, should he perform ghusl and repeat all of his prayers since the last time he slept? If he thinks that it happened before his most recent sleep, he should repeat all of his prayers since the supposed time of his ejaculation.