Midrasch zu Bamidbar 17:15

Just as one reaps row by row, so the plague acted upon the people (in rotation). Thereupon Aaron took as Moses spoke and ran into the midst of the assembly; and behold, the plague was begun (Num. 7:12). He found the angel standing among them and afflicting them. Aaron confronted him and would not permit him to continue. He forced him to stand idly among the dead. He said to Aaron: “Let me go that I may complete my task.” Aaron replied: “Moses sent me, and the Holy One, blessed be He, sent you, and both Moses and the Holy One, blessed be He, are in the Tent of Testimony; let us go to them.” The angel refused to go, and so Aaron seized him by his loins and dragged him in, as it is said: And Aaron returned unto Moses unto the door of the Tent of Meeting, and the plague was stayed (Num. 17:15).