Quran 4:25

وَمَن لَّمۡ يَسۡتَطِعۡ مِنكُمۡ طَوۡلاً أَن يَنڪِحَ ٱلۡمُحۡصَنَـٰتِ ٱلۡمُؤۡمِنَـٰتِ فَمِن مَّا مَلَكَتۡ أَيۡمَـٰنُكُم مِّن فَتَيَـٰتِكُمُ ٱلۡمُؤۡمِنَـٰتِ‌ۚ وَٱللَّهُ أَعۡلَمُ بِإِيمَـٰنِكُم‌ۚ بَعۡضُكُم مِّنۢ بَعۡضٍ۬‌ۚ فَٱنكِحُوهُنَّ بِإِذۡنِ أَهۡلِهِنَّ وَءَاتُوهُنَّ أُجُورَهُنَّ بِٱلۡمَعۡرُوفِ مُحۡصَنَـٰتٍ غَيۡرَ مُسَـٰفِحَـٰتٍ۬ وَلَا مُتَّخِذَٲتِ أَخۡدَانٍ۬‌ۚ فَإِذَآ أُحۡصِنَّ فَإِنۡ أَتَيۡنَ بِفَـٰحِشَةٍ۬ فَعَلَيۡہِنَّ نِصۡفُ مَا عَلَى ٱلۡمُحۡصَنَـٰتِ مِنَ ٱلۡعَذَابِ‌ۚ ذَٲلِكَ لِمَنۡ خَشِىَ ٱلۡعَنَتَ مِنكُمۡ‌ۚ وَأَن تَصۡبِرُواْ خَيۡرٌ۬ لَّكُمۡ‌ۗ وَٱللَّهُ غَفُورٌ۬ رَّحِيمٌ۬ ٢٥

Those among you who cannot afford to marry free believing women, may marry believing slave women. ALLAH knows best about your belief, and you are equal to one another, as far as belief is concerned. You shall obtain permission from their guardians before you marry them, and pay them their due dowry equitably. They shall maintain moral behavior, by not committing adultery, or having secret lovers. Once they are freed through marriage, if they commit adultery, their punishment shall be half of that for the free women. Marrying a slave shall be a last resort for those unable to wait. To be patient is better for you. ALLAH is Forgiver, Most Merciful.