Musannaf Ibn Abi Shayba 1:981

Abu al-Ahwas narrated to us, from Simak, who said: I said to al-Hasan al-Basri, "What is your opinion regarding a man when he emits madhy (pre-seminal fluid); what should he do?" He said, "Every stallion emits madhy. So when that happens, let him wash his private part."