Imam Malik (may Allah have mercy on him) was informed that Sayyiduna Abdullah bin Abbas and Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) were asked about a man who had a free woman as his wife and wished to marry a slave woman in addition to her; they both disliked that he should combine the two.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14005