Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 605
Abu Sa'id al-Khudri reported: The Messenger of Allah (may peace be upon him) said: When anyone amongst you has sexual intercourse with his wife and then he intends to repeat it, he should perform ablution. In the hadith transmitted by Abu Bakr. (the words are):" Between the two (acts) there should be an ablution," or he (the narrator) said:" Then he intended that it should be repeated."