Alqamah and al-Aswad (may Allah have mercy on them) said that a guest came to Sayyidah Aishah (may Allah be pleased with her), and in the morning he was washing his clothes. Sayyidah Aishah (may Allah be pleased with her) said: It would have been enough for you to wash only that spot. If you could not see the spot, you should have sprinkled water around it. When I saw semen on the clothes of the Prophet (peace and blessings be upon him), I would scrape it off, and he (peace and blessings be upon him) would pray in them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4165