Sahih Muslim Book 3, Hadith Number 677.

Chapter : Emission of semen makes bath obligatory.

Ubayy Ibn Ka’b reported: I asked the Messenger of Allah (may peace be upon him) about a man who has sexual intercourse with his wife, but leaves her before orgasm. Upon this he (the Holy Prophet) said: He should wash the secretion of his wife, and then perform ablution and offer prayer.