Maliks Muwatta Book 36, Hadith Number 24.

Yahya said that Malik related from Ibn Shihab from Salim ibn Abdullah ibn Umar from his father that Umar ibn al-Khattab said, “What’s the matter with men who have intercourse with their slave-girls and then dismiss them? No slave-girl comes to me whose master confesses that he has had intercourse with her but that I connect her child to him, whether or not he has practised coitus interruptus or stopped having intercourse with her.”