Midrasch zu Tehillim 99:7

7) (Vayikra 9:7): "And Moses said to Aaron: Draw near to the altar, etc." (Of) this (moment, Moses inducting Aaron into the service on the eighth day of the miluim, to serve with him in the priesthood, [Moses having served alone the first seven days]), it is written (Tehillim 99:6): "Moses and Aaron (the elect of) His priests, and Samuel (among the elect of) the invokers of His name. They called to the L–rd and He answered them. (Tehillim 99:7) In a pillar of cloud He spoke to them" — whereby we are taught that the three were on a par.