From Sahih Muslim (Hadith 707):
Abu Sa'id al-Khudri reported:The Messenger of Allah (ﷺ) 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
Reference: Muslim 707