Midrasch zu Schemuel II 22:27

R. Chelbo questioned R. Samuel h. Nachmeni: "What is the reason that Jacob took away the privilege of the first-born from Reuben and gave it to Joseph?" You ask for the reason? Does not the Scripture state the reason When he defiled his father's bed? "What I ask, is this: Why did he give it to Joseph?" and he rejoined: "I will tell you a parable to which this case is similar: There was one who had raised an orphan in his house. At a later period the orphan became rich, and thought, I will recompense my benefactor." R. Chelbo said to him: "And how would it have been had Reuben not sinned? Then Jacob would have given nothing to Joseph?" R. Samuel b. Nachmeni replied: "Your teacher, R. Jonathan, did not explain it so, but as follows: The first-born was destined to come from Rachel, as it is written (Ib. 37) These are the generations of Jacob and Joseph. But Leah was preferred by virtue of her prayers. Nevertheless, because of Rachel's discretion, the Holy One, praised be He! returned it to her." And in what consisted Rachel's discretion? As it is written (Ib. 12) And Jacob told Rachel that he was her father's brother, and that he was Rebekah's son. The brother of her father? Was he not the son of her father's sister? It was thus: He asked her whether she would marry him. She said, "Yea, but my father is very shrewd, and you cannot persuade him." And to the question: "What does it mean?" She answered: "I have a sister who is older than me, and my father will not give me to you while she is not married." Then he said: "I am his brother in shrewdness." She then asked him: "Is it, then, allowed to the upright to be shrewd?" And he answered: "Yea, as (II Sam. 22, 27) With the pure thou wilt show thyself pure, and with the perverse thou wilt wage a contest." And then he furnished her with some signs, that when she should be brought to him he would ask her for these signs, and he might be sure that she was not exchanged for Leah. Thereafter, when Leah was about to be brought to him instead of Rachel, the latter said to herself: "Now my sister will he ashamed." She went and confided the signs to her. And this is what is written (Gen. 29, 25) And it came to pass that in the morning, behold, it was Leah. From which it is to be inferred that until the morning he did not know that she was Leah, because of the signs which Leah received from Rachel.

Variantly: "a crooked and twisted generation": Moses hereby says to Israel: "With the measure that you have measured I have measured you." And thus is it written (II Samuel 22:27) "With the pure You are pure, and with the crooked You are crafty."