Midrasch zu Bereschit 20:8

Another comment on this matter. Moses said to the Holy One, blessed be He: But perhaps they will not believe me, nor hearken to my voice (Exod. 4:1). And He said: What is that in thy hand? (ibid., v. 2). The word is written as mazeh (“what is this?”), but it may be read as mi-zeh (“with this”): “With this that is in your hand you will be punished,³¹Indicating that he would be punished for striking the rock with the rod (see Num. 20:7–13). for you have spoken slanderously against My children, just as the serpent spoke slanderously.” And He said: For God doth know (Gen. 3:5) that My son’s children are believers, and the descendants of believers. They are believers, because it is written: And the people believed (Exod. 4:31), and the descendants of believers, because it is written: And he believed in the Lord (Gen. 15:6). Just as I smote the snake with leprosy, so you shall be smitten by it, place your hand in your bosom. And he put his hand into his bosom; and behold, when he withdrew it, it was leprous and as white as snow (Exod. 4:6). And if it shall come to pass that they do not believe you (ibid., v. , 8), then you shall smite the waters of the Nile and they shall turn into blood. This was also a sign to him that he would be judged in the future because of water, as it is said: Out of the rock, etc. (Gen. 20:8).

(Gen. 20:8:) THEN ABIMELECH AROSE EARLY IN THE MORNING…. Having observed the morning, when would he arise? Immediately (ibid.), HE AROSE AND CALLED ALL HIS SERVANTS. He said to them: The Holy One said thus and so to me in a dream at night. They said to him: Do not be afraid. Dreams speak empty talk; so would you be afraid of dreams? When they had said this to him, the Holy One said to him (in vs. 7): BUT IF YOU DO NOT RESTORE HER, KNOW THAT YOU SHALL DIE. (Gen. 8f.:) SO THE MEN WERE VERY AFRAID. THEN ABIMELECH CALLED ABRAHAM. Abraham said to Abimelech (in vss. 11f.): BECAUSE I SAID: SURELY THE FEAR OF GOD IS NOT IN THIS PLACE … AND MOREOVER SHE IS INDEED MY SISTER, MY FATHER'S DAUGHTER, BUT NOT THE DAUGHTER OF MY MOTHER. Hence they have said: A gentile star worshiper has no father.¹⁰⁴I.e., one does not recognize the father’s family because a father’s daughter is permitted him, but a mother’s daughter is not. A marriage to her would be incestuous. So Gen. R. 18:5; 52:11; see yYev. 18:2 (12a); Yev. 98ab; Sanh. 57b-58a.