Ja'far bin Burqan (may Allah have mercy on him) said: I asked Ata (may Allah have mercy on him) about 'azl (withdrawal), and he said: with a free woman, according to her consent, whereas with a slave woman, according to your own wish.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14327