What did Joseph have hidden in Benjamin's sack?

Answer: His silver cup

Joseph's steward put his silver cup in the youngest one's sack, and when the sacks were searched it was found in Benjamin's.

“Put my cup, the silver one, in the mouth of the youngest one’s sack, along with the silver for his grain.” So the steward did as Joseph had instructed. The steward searched, beginning with the oldest and ending with the youngest—and the cup was found in Benjamin’s sack.”— Genesis 44:2, 44:12 (BSB)