Mujahid (may Allah have mercy on him) said: "And whoever among you does not have the means" [Surah an-Nisa: 25], "whoever among you is unable to marry": the one who has no wealth, that is, whoever is unable to marry a free believing woman should marry a believing slave woman. "And that you be patient is better for you" [Surah an-Nisa: 25], that is, (being patient) from marrying a slave woman is "better for you", and it is lawful.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13992