Allah ti parla (Ep. 7): Sura 4, versetti 24-25

“Also forbidden are married women—except female captives in your possession. This is Allah’s commandment to you. Lawful to you are all beyond these—as long as you seek them with your wealth in a legal marriage, not in fornication. Give those you have consummated marriage with their due dowries. It is permissible to be mutually gracious regarding the set dowry. Surely Allah is All-Knowing, All-Wise.
But if any of you cannot afford to marry a free believing woman, then let him marry a believing bondwoman possessed by one of you. Allah knows best the state of your faith and theirs. You are from one another. So marry them with the permission of their owners, giving them their dowry in fairness, if they are chaste, neither promiscuous nor having secret affairs. If they commit indecency after marriage, they receive half the punishment of free women. This is for those of you who fear falling into sin. But if you are patient, it is better for you. And Allah is All-Forgiving, Most Merciful.”