Genesis 26:9

And Abimelech called Isaac
Sent a messenger to desire him to come to him: and said, behold, of a surety she [is] thy 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 saidst thou, 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.