Albert Barnes Commentary Numbers 17:10

"And Jehovah said unto Moses, Put back the rod of Aaron before the testimony, to be kept for a token against the children of rebellion; that thou mayest make an end of their murmurings against me, that they die not." — Numbers 17:10 (ASV)

The testimony - that is, the two tables of the Law; compare Exodus 25:16 (see note). No doubt the rod lay in front of the tables within the ark. In the days of Solomon (1 Kings 8:9), there was nothing in the ark except the Two tables. Aaron’s rod was probably lost when the ark was taken by the Philistines.