There are three things kept in the ark of the covenant/ ark of testimony. The 10 commandments written on stone tablets, a jar with an omer of manna, and Aaron’s staff. Aaron’s staff is not actually in the ark, but kept with it in order to remind the Israelites who God has chosen to do his work. Here is that story.

Numbers 17

The LORD spoke to Moses, saying:

2 “Speak to the children of Israel, and take from each of them a staff according to the house of their fathers, 12 staffs representing the 12 ancestral houses, and write every man’s name on his staff. 

3 You shall write Aaron’s name on the staff of Levi: for one staff shall be for the head of the house of their fathers.

4 And you shall lay them up in the tent of meeting before the testimony, where I will meet with you.

5 The man’s staff that I choose, will sprout, and I will make an end of the grumblings of the children of Israel that grumble against you.”

6 Moses spoke to the children of Israel, and every one of their princes gave him a staff, one for each house, according to their fathers’ houses, 12 staffs: and the staff of Aaron was among their staffs..

7 And Moses laid the staffs before the LORD in the tabernacle of witness.

8 The next morning Moses went into the tabernacle of witness; and Aaron’s staff for the house of Levi had sprouted, formed buds, blossomed and yielded almonds.

9 Moses brought out all the staffs from before the LORD to all the children of Israel: and they looked, and every man took his staff.

10 The LORD said to Moses “Bring Aaron’s staff again before the testimony, to be kept as a sign against the rebels; and you shall take away their grumblings from me so they do not die.” 

11 Moses did so: as the LORD commanded him, so he did. 

12 And the children of Israel spoke to Moses, saying “Behold, we die, we perish, we all perish.

13 Anyone who comes near the tabernacle of the LORD shall die: will we all perish? 

See also: Numbers 17 in ESV, HCSB, NASB and NIV via Bible Gateway.