Numbers 33:38 in the King James Version
And Aaron the priest went up into mount Hor at the commandment of the LORD, and died there, in the fortieth year after the children of Israel were come out of the land of Egypt, in the first day of the fifth month.
Context
37 They traveled from Kadesh, and encamped in Mount Hor, in the edge of the land of Edom.
38 Aaron the priest went up into Mount Hor at the commandment of the LORD and died there, in the fortieth year after the children of Israel had come out of the land of Egypt, in the fifth month, on the first day of the month.
39 Aaron was one hundred twenty-three years old when he died in Mount Hor.