Midrasch zu Schemot 32:21

Why did He elevate Aaron and his sons to that holy station? R. Mani of Shaab and R. Joshua of Sikhnin said in the name of R. Levi: When the Israelites pleaded with him to erect the golden calf, they said to Aaron: Up, make us a god (Exod. 32:1). Thereupon Hur the son of Caleb arose and rebuked them. They attacked him and killed him. Aaron beheld what had happened and feared (vayir’a) for his own life, as it is said: And when Aaron saw (vaya’r) he built an altar before it (ibid., v. 5). You know that they killed Hur, for Moses told them before he ascended the mountain: Behold, Aaron and Hur are with you; whosoever hath a cause, let him come near unto them (ibid. 24:14), but after he descended, you find that he did not refer to Hur either as living or as dead. What does Scripture say: And Moses said to Aaron: “What did this people unto thee, that thou hast brought a great sin upon them?” (ibid. 32:21). Obviously they must have killed Hur.