Albert Barnes Commentary Numbers 20:8

"Take the rod, and assemble the congregation, thou, and Aaron thy brother, and speak ye unto the rock before their eyes, that it give forth its water; and thou shalt bring forth to them water out of the rock; so thou shalt give the congregation and their cattle drink." — Numbers 20:8 (ASV)

Take the rod - That with which the miracles in Egypt had been performed (Exodus 7:8 and following; Exodus 7:19 and following; Exodus 8:5 and following, etc.), and which had been used on a similar occasion at Rephidim (Exodus 17:5 and following). This rod, as the memorial of so many divine interpositions, was naturally laid up in the tabernacle, and is accordingly described in Numbers 20:9 as taken by Moses from before the Lord.