Numbers 33:37-39

37They travelled from Kadesh and they made their camp at Hor mountain. It is on the border of Edom land.

38The Lord told Aaron that he must go up Hor mountain. Aaron went up and he died there. That was in the 40th year after the Israelites had left Egypt. He died on the first day of the fifth month.

39Aaron was 123 years old when he died on Hor mountain.