Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 372
Narated By 'Aisha : I used to rub off the semen from the clothe of the Apostle of Allah (pbuh). He then would pray in it.
Abu Dawud said: Mughairah, Abu Ma'shar and wasil also narrated it to the same effect.