Genesis 26:7-9

7And the men of the place asked about his wife. And he said, She is my sister; for he feared to say, my wife, [saying to himself,] Lest the men of the place slay me on account of Rebecca — because she was fair in countenance.

8And it came to pass when he had been there some time, that Abimelech the king of the Philistines looked out of the window, and saw, and behold, Isaac was dallying with Rebecca his wife.

9Then Abimelech called Isaac, and said, Behold, she is certainly thy wife; and how saidst thou, She is my sister? and Isaac said to him, Because I said, Lest I die on account of her.