Numbers 33:38
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. 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.
Aaron died on Mount Hor. He ended his life there because he was barred from entering the promised land for the sin he shared with Moses in bringing water from the rock at Meribah. Moses together with Aaron's sons buried him on the mountain, so that the people would not make excessive ceremony over his body and fall into idolatry.