An-Nisa · 4:25

25. Whoever among you cannot marry a free woman, because he does not have enough wealth, is allowed to marry a believing slave owned by someone else. Allah knows the reality of your faith and your situation, and you are equal in your faith and humanity, so do not look down on marrying them. Marry them with the permission of their owners, and give them their dowries without reducing the amount or delaying it. This is if they are chaste and do not commit sexual offences publicly and do not take lovers secretly. If you marry them and then they have illicit sexual relations, their punishment is half that of a free woman: fifty lashes, and not stoning, which is the punishment for chaste women. Marrying chaste believing slave women is a concession for those who fear falling into illicit sexual relations, and who are not able to marry free women. Patience is preferable to marrying slave women though, so that the children do not become slaves. Allah is Forgiving and Compassionate to those of His servants who repent to Him, and it is from His mercy that He allowed marriage to slaves in the case of not being able to marry free women, for those who fear falling into illicit sexual relations.