Genesis 26:7
And the men of the place asked him of his wife; and he said, She is my sister: for he feared to say, She is my wife; lest, said he, the men of the place should kill me for Rebekah; because she was fair to look upon. And the men of the place asked him of his wife; and he said, She is my sister: for he feared to say, She is my wife; lest, said he, the men of the place should kill me for Rebekah; because she was fair to look upon.
Genesis 26:7 belongs to the account of Isaac's time in Philistia, in Gerar, in the country of the Philistines. It is part of that time when he became prosperous and the Lord blessed him. The account is part of God's special providence over Isaac as one of the progenitors of the promised Seed. God's dealing with Isaac in this account is an example of His instructing His people after sin by an audible voice, whereas before sin there was face-to-face communion. Isaac's life in Philistia as presented in the chapter is a model of faithful living and missionary work.
The verse records that Isaac said Rebekah was his sister, denying and concealing that she was his wife, because she was beautiful and he feared the men of the place, the men of Gerar, would kill him to take her. In this claim Isaac was following his father's example. Abimelech, king of Gerar, took Rebekah supposing her to be unmarried, then rebuked Isaac because one of the people might have lain with her and Isaac would have brought guilt upon them. God used Abimelech to rebuke Isaac over this deception, as He had earlier used him to rebuke Abraham, and these encounters were opportunities for Abimelech and his people to know God.
The incident shows that taking another man's wife and sexual relations with her were already known to be sinful adultery bringing guilt. This proves the seventh commandment was known and binding long before Sinai.