Sahih Muslim Book 8, Hadith Number 3384.

Jabir b. ‘Abdullah (Allah be pleased with them) reported that a person asked Allah’s Apostle (May peace be upon him) saying: I have a slave-girl and I practise ‘azl with her, whereupon Allah’s Messenger (May peace be upon him) said: This cannot prevent that which Allah has decreed. The person then came (after some time) and said: Messenger of Allah, the slave-girl about whom I talked to you has conceived, whereupon Allah’s Messenger (May peace be upon him) said: I am the servant of Allah and His Messenger.