Sahih Muslim Book 3, Hadith Number 680.

Chapter : Emission of semen makes bath obligatory.

Zaid b. Khalid al-Jubani reported that he asked Uthman b. ‘Affan: What is your opinion about the man who has sexual intercourse with his wife, but does not experience orgasm? Uthman said: He should perform ablution as he does for prayer, and wash his organ. ‘Uthmin also said: I have heard it from the Messenger of Allah (may peace be upon him).