حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، وَعَبْدَ اللَّهِ بْنَ عُمَرُ، سُئِلاَ عَنْ رَجُلٍ، كَانَتْ تَحْتَهُ امْرَأَةٌ حُرَّةٌ فَأَرَادَ أَنْ يَنْكِحَ عَلَيْهَا أَمَةً فَكَرِهَا أَنْ يَجْمَعَ بَيْنَهُمَا ‏.‏

Yahya related to me from Malik that he had heard that Abdullahibn Abbas and Abdullah ibn Umar were asked about a man who had a freewoman as a wife and then wanted to marry a slave-girl. Theydisapproved that he should combine the two of them. Translation Not Available

Book Ref: Muwatta Imam Malik Book 28 Hadith 1122
Web Ref:  Muwatta Imam Malik Book 28 Hadith 28