Sahih Muslim Book - 2, Hadith - 672

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ عَنْ عَمْرِو بْنِ مَيْمُونٍ قَالَ سَأَلْتُ سُلَيْمَانَ بْنَ يَسَارٍ عَنْ الْمَنِيِّ يُصِيبُ ثَوْبَ الرَّجُلِ أَيَغْسِلُهُ أَمْ يَغْسِلُ الثَّوْبَ فَقَالَ أَخْبَرَتْنِي عَائِشَةُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَغْسِلُ الْمَنِيَّ ثُمَّ يَخْرُجُ إِلَى الصَّلَاةِ فِي ذَلِكَ الثَّوْبِ وَأَنَا أَنْظُرُ إِلَى أَثَرِ الْغَسْلِ فِيهِ.

It was narrated that 'Amr bin Maimun said: "I asked Sulaiman bin Yasar about semen that gets onto a man's garment, should he wash it or wash the (whole) garment?" He said: "'Aishah told me that the Messenger of Allah (s.a.w) used to wash the semen off, then go out to perform Salat in that garment, and I could see the traces of washing on it."