The Catechism in Questions & Answers

343. What is the significance of the Sixth Commandment in the Old Testament?

Already at the time of the Old Testament, marriage was considered a covenant protected by God and blessed through prayer. At that time, adultery was punished by death.

“And after they were both shut in together, Tobias rose out of the bed, and said, ‘Sister, arise, and let us pray that God will have pity on us.’ Then began Tobias to say. ‘Blessed art Thou, O God of our fathers, and blessed is Thy holy and glorious name for ever; let the heavens bless Thee and all Thy creatures.”
Tobit 8: 4-5