Genesis 42:34
And bring your youngest brother unto me: then shall I know that ye are no spies, but that ye are true men: so will I deliver you your brother, and ye shall traffick in the land. And bring your youngest brother unto me: then shall I know that ye are no spies, but that ye are true men: so will I deliver you your brother, and ye shall traffick in the land.
Genesis 42:34 belongs to Joseph's test of his brothers, which began when he accused them of being spies and they answered that their youngest brother was with their father and another brother was gone. Joseph makes bringing their youngest brother Benjamin back to him the proof that they are honest, true men and not spies.
Until then, one brother must remain bound in Egypt while the rest take food for their households, which the brothers reported to Jacob by saying the lord of the land would not believe their word. When they return with their youngest brother, Joseph will release to them the bound brother Simeon. He will then let them trade in Egypt.