Sahih Bukhari Volume 001, Book 004, Hadith Number 180.
Narated By Abu Said Al-Khud : Allah's Apostle sent for a Ansari man who came with water dropping from his head. The Prophet said, "Perhaps we have forced you to hurry up, haven't we?" The Ansari replied, "Yes." Allah's Apostle further said, "If you are forced to hurry up (during intercourse) or you do not discharge then ablution is due on you (This order was cancelled later on, i.e. one has to take a bath).