Allah Almighty says: "And whoever among you does not have the financial means to marry free believing women, then (let him marry) from the believing slave girls in your possession; and Allah knows best your faith." [Surah an-Nisa: 25] Up to (Allah Almighty's) words: "That (permission) is for whoever among you fears falling into sin (if he does not marry)." [Surah an-Nisa: 25]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q13991