Musnad Ahmad 1153:685

'Affan narrated to us, he said: Mahdi narrated to us, he said: Wasil al-Ahdab narrated to us, from Ibrahim al-Nakha'i, from al-Aswad ibn Yazid, who said: 'Aisha, the Mother of the Believers, saw me washing the trace of seminal fluid that had gotten onto my garment. She said: "What is this?" I said: "Seminal fluid that got onto my garment." She said: "I have seen us, and it would get onto the garment of the Messenger of Allah, may Allah bless him and grant him peace, and he would do nothing more than say [do] this with it." And Mahdi described it: "He rubbed his hand against the other."