From Sahih Muslim (Hadith 672):
Amr b. Maimun said:I asked Sulaiman b. Yasar whether the semen that gets on to the garment of a person should be washed or not. He replied: A'isha told me: The Messenger of Allah (ﷺ) washed the semen, and then went out for prayer in that very garment and I saw the mark of washing on it
Reference: Muslim 672