Genesis 19:35 (KJV)

And they made their father drink wine that night: and the firstborn went in, and lay with her father; and he perceived not when she lay down, nor when she arose.

And it came to pass on the morrow, that the firstborn said unto the younger, Behold, I lay yesternight with my father: let us make him drink wine this night also; and go thou in, and lie with him, that we may preserve seed of our father.

And they made their father drink wine that night also: and the younger arose, and lay with him; and he perceived not when she lay down, nor when she arose.

Thus were both the daughters of Lot with child by their father.

And the firstborn bare a son, and called his name Moab: the same is the father of the Moabites unto this day.