Sahih Muslim Book 003, Hadith Number 0684.
Chapter : Abrogation of (the command that) bath is obligatory(only) because of seminal emission and instead contact of the circumcised parts makes bath obligatory.
Abu Musa reported: There cropped up a difference of opinion between a group of Muhajirs (Emigrants and a group of Ansar (Helpers) (and the point of dispute was) that the Ansar said: The bath (because of sexual intercourse) becomes obligatory only-when the semen spurts out or ejaculates. But the Muhajirs said: When a man has sexual intercourse (with the woman), a bath becomes obligatory (no matter whether or not there is seminal emission or ejaculation). Abu Musa said: Well, I satisfy you on this (issue). He (Abu Musa, the narrator) said: I got up (and went) to 'Aisha and sought her permission and it was granted, and I said to her: O Mother, or Mother of the Faithful, I want to ask you about a matter on which I feel shy. She said: Don't feel shy of asking me about a thing which you can ask your mother, who gave you birth, for I am too your mother. Upon this I said: What makes a bath obligatory for a person? She replied: You have come across one well informed! The Messenger of Allah (may peace be upon him) said: When anyone sits amidst four parts (of the woman) and the circumcised parts touch each other a bath becomes obligatory.