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
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
Amr bin Maymun (may Allah have mercy on him) said: I asked Sulayman bin Yasar (may Allah have mercy on him) about semen that gets onto clothes: should it be washed, or the clothes? He said: Aishah (may Allah be pleased with her) said that the Prophet (peace and blessings be upon him) would wash the semen and go out to the prayer in those same clothes, and I would see the marks of washing on his clothes.
The context of the hadith of Muhammad bin Bishr indicates that the sweat of a junub person is pure, and that it is not obligatory to wash the clothes in which he became junub; the semen is washed only for cleanliness, just as spittle and the like are washed from clothes for cleanliness, not because of impurity.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4181