The testimony - i. e. the two tables of the Law; compare Exodus 25:16 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 save the Two tables. Aaron’s rod was probably lost when the ark was taken by the Philistines.
Related Commentaries of Numbers 17:10
Numbers 17:10
10 And the LORD said unto Moses, Bring Aaron's rod again before the testimony, to be kept for a token against the rebels;b and thou shalt quite take away their murmurings from me, that they die not.