Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 682
Abu Huraira reported: The Apostle of Allah (may peace be upon him) said: When a man has sexual intercourse, bathing becomes obligatory (both for the male and the female). In the hadith of Matar the words are: Even if there is no orgasm. Zuhair has narrated it with a minor alteration of words.