Numbers 20:25

Take Aaron and Eleazar his son, and bring them up unto mount Hor:

God commanded Moses to take Aaron and Eleazar his son and bring them up unto Mount Hor. Moses, Aaron's brother, went up with Aaron, accompanied by Eleazar his son. The ascent was Aaron's final journey to the place where he would die on the mount and be buried there, barred from entering the promised land. That barring was punishment for the sin Aaron shared with Moses at Meribah when bringing water from the rock.

On the mount Moses was to remove Aaron's priestly garments from him and place them on Eleazar. By that putting of the garments on Eleazar, God appointed him to succeed Aaron in the priesthood and sacred office. For Moses, removing the sacerdotal robes from his brother was a painful duty.

Moses and Eleazar witnessed Aaron's death on Mount Hor, and Moses buried him in the mount. Those who held that Moses was joined by Aaron's sons teach that Moses with Aaron's sons buried him on the mount. Burial on the mount kept the people from making too great ceremony over his body and falling into idolatry.