Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said regarding the words of Allah Almighty, "And whoever among you does not have the means to marry free believing women, then (let him marry) from the believing slave girls in your possession" [Surah an-Nisa: 25]: Whoever does not have the means to marry free women should marry from among the Muslim slave women. "That is for whoever among you fears falling into hardship" [Surah an-Nisa: 25]: this is sin. It is not permissible for any free man to marry a slave woman unless he is unable to marry a free woman and he fears sin. "And that you be patient" [Surah an-Nisa: 25], that is, refraining from marrying slave women, "is better for you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13991