Sahih Muslim Book 003, Hadith Number 0676.
Chapter : Emission of semen makes bath obligatory.
Abu Sa'id al-Khudri reported: The Messenger of Allah (may peace be upon him) happened to pass by (the house) of a man amongst the Ansar, and he sent for him. He came out and water was trickling down from his head. Upon this he (the Holy Prophet) said: Perhaps we put you to haste. He said: Yes. Messenger of Allah. He (the Holy Prophet) said: When you made haste or semen is not emitted, bathing is not obligatory for you, but ablution is binding. Ibn Bashir has narrated it with a minor alteration.