Musnad Ahmad 1153:1967

Yazid narrated to us, he said: 'Amr ibn Maymun informed us, he said: Sulayman ibn Yasar informed me, he said: 'Aisha informed me that when semen would get on the garment of the Messenger of Allah, may Allah bless him and grant him peace, he would wash what had touched his garment, then he would go out to prayer, and I would be looking at the spot on his garment, that being from the trace of the washing.