Musnad Ahmad 31:174

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Hakam, from Dhakwan, from Abu Sa'id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) passed by a man from the Ansar, so he sent for him, and he came out while his head was dripping. He said to him: "Perhaps we have hastened you?" He said: "Yes, O Messenger of Allah." He said: "If you are hastened, or if you do not emit (semen), then there is no ghusl (full ritual bath) upon you; upon you is wudu (ablution)."