Musnad Ahmad 1153:2246

Abu Qatan narrated to us, he said: Abbad ibn Mansur narrated to us, from al-Qasim ibn Muhammad, from Aisha, she said: "I used to rub the semen off the garment of the Messenger of Allah, may Allah bless him and grant him peace, then I would not wash it." Abu Qatan said: She said once: "its trace." And she said once: "its place."