Genesis 42:15-20

15And this is how you will be tested: As surely as Pharaoh lives, you will not leave this place unless your youngest brother comes here. 16Send one of your number to get your brother; the rest of you will be kept in prison, so that your words may be tested to see if you are telling the truth. If you are not, then as surely as Pharaoh lives, you are spies!”

17And he put them all in custody for three days. 18On the third day, Joseph said to them, “Do this and you will live, for I fear God: 19If you are honest men, let one of your brothers stay here in prison, while the rest of you go and take grain back for your starving households.

20But you must bring your youngest brother to me, so that your words may be verified and that you may not die.” This they proceeded to do.