Sayyidah Aishah (may Allah be pleased with her) said: When semen got onto his (peace and blessings be upon him) clothes, he (peace and blessings be upon him) would wash only the part of the garment that had semen on it, then he (peace and blessings be upon him) would go out to the prayer, and I would see the marks of the stains in the washed clothes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4179