Genesis 42:16

13 Then they said, We your servants are twelve brothers, sons of one man in the land of Canaan; the youngest of us is now with our father, and one is dead.

14 And Joseph said, It is as I said; you have come with some secret purpose;

15 But in this way will you be put to the test: by the life of Pharaoh, you will not go away from this place till your youngest brother comes here.

16 Send one of your number to get your brother, and the rest of you will be kept in prison, so that your words may be tested to see if you are true; if not, by the life of Pharaoh, your purpose is certainly secret.

17 So he put them in prison for three days.

18 And on the third day Joseph said to them, Do this, if you would keep your lives: for I am a god-fearing man:

19 If you are true men, let one of you be kept in prison, while you go and take grain for the needs of your families;