Salim bin Abdullah narrates from his father that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: What is the matter with men who have intercourse with their slave women and then practise withdrawal ('azl)? Any slave woman who comes (to me) whose master acknowledges having had intercourse with her, I will attach the child to him. After this, practise withdrawal or leave it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15374