Sa'id bin Jubayr (may Allah have mercy on him) said regarding this verse: "Tawl" means wealth; when he does not have the means to marry a free woman, then he may marry a slave woman. And regarding the words, "And that you be patient is better for you" [Surah an-Nisa: 25], he said: (being patient) from marrying slave women. And "al-'anat" means zina (fornication).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13993