Genesis 26:8

And it came to pass, when he had been there a long time, that Abimelech, king of the Philistines, looked out at a window, and saw, and, behold, Isaac was sporting with Rebekah, his wife. By chance Abimelech, looking out of a window of his palace, became witness of the fact that Isaac, who probably occupied a house with its open court nearby, was treating his wife Rebekah with such familiar endearments as are altogether in order between husband and wife, but would hardly be considered permissible between brother and sister.