Muwatta Malik 1119

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 Abdullah ibn Abbas and Abdullah ibn Umar were asked about a man who had a free woman as a wife and then wanted to marry a slave-girl. They disapproved that he should combine the two of them