Sulayman bin Yasar (may Allah have mercy on him) said: I asked Aishah (may Allah be pleased with her) about semen that gets onto clothes, and she said: I used to wash semen off the clothes of the Prophet (peace and blessings be upon him), then he (peace and blessings be upon him) would go out to the prayer, and there would be stains on the washed clothes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4180