Targum zu Bamidbar 20:29

And when the soul of Aharon was at rest, the Cloud of Glory was lifted up on the first day of the month Ab; and all the congregation beheld Mosheh come down from the mountain with rent garments; and he wept and said, Woe unto me, for thee, my brother Aharon, the pillar of Israel's prayers! And they too wept for Aharon thirty days, the men and the women of Israel.

And all the congregation beheld Mosheh come down from the height of the mountain, with garments rent and dust upon his head, weeping and saying, Woe unto me, for thee, my brother Aharon, the pillar of the prayers of the sons of Israel, who madest atonement for them once every year! In that hour the sons of Israel believed that Aharon was dead; and all the congregation of the children of Israel wept for Aharon thirty days.