Abu al-Akhdar narrates from Sayyiduna Ammar (may Allah be pleased with him) that he regarded as disliked with regard to slave-women the same thing he disliked with regard to free women (that is, in establishing relations), except for the specific number. Imam al-Shafi'i (may Allah have mercy on him) said: This saying of Ammar (may Allah be pleased with him) is in accordance with the Qur'an, and we act upon it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13928