Purity: Finding Semen on Clothing

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 353:

If a person sees semen on his clothes and knows that it has come from himself and he has not performed ghusl, he must perform ghusl and make up [i.e. perform as qaḍāʾ] those prayers that he is certain of having performed while he was junub. However, it is not necessary for him to make up those prayers that he deems only probable of having performed before ejaculation of the semen.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)