The Women - 4:16 Punish the couple among you who commit adultery. If they repent and reform, forsake them. Allah is All-accepting, All-merciful.
Related:
The Light/24:2 Strike each of the man and woman who commits adultery with a hundred strokes of the gavel. If you believe in Allah and the Last Day, your pity for them should not prevent you from applying Allah´s law. Let a group of believers witness their punishment.

The Women/4:25 Those of you who cannot afford to marry free women, let them marry those to whom your believing oaths/agreements entitle you. Allah is the Best Knower of your faith. You are equal to each other. Marry them with the permission of their owners and pay them their mahr accordingly. Let them live chaste lives, not committing adultery and not taking secret friends. If they commit adultery after they have gained their freedom through marriage, they should be punished with half the punishment given to free women. This is for those of you who fear sin. It is better for you if you are patient. Allah is Forgiving, Merciful.