John Gill Commentary Genesis 26:9

"And Abimelech called Isaac, and said, Behold, of a surety she is thy wife. And how saidst thou, She is my sister? And Isaac said unto him, Because I said, Lest I die because of her." — Genesis 26:9 (ASV)

And Abimelech called Isaac Sent a messenger to desire him to come to him: and said, behold, of a surety she [is] your wife: and then perhaps told him of what he had observed that passed between them; which was a clear proof that they must be man and wife, or he would never have took such liberties with her: and how did you say, she [is] my sister? what reason hadst thou for it? what could induce thee to say so? and Isaac said unto him; not alleging, as Abraham did, any relation that was between them before marriage: because I said; that is, within himself, for, he did not speak it out to others: lest I die for her; for her sake, that another might have and enjoy her; it was fear of losing his life that led him to take such a step, and give out that she was his sister.