Musnad Ahmad 31:893

'Abd al-Razzaq narrated to us, al-Thawri informed us, from al-A'mash, from Dhakwan, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you hastens or experiences a drought (i.e., does not ejaculate), then he must not perform ghusl."