"and Aaron and all the elders of Israel came to eat bread with Moses' father-in-law before G–d": And where did Moses go? Did he not go out to him in the beginning? viz. (Ibid. 3) "and Moses went out to his father-in-law." We are hereby apprised that he stood and waited upon them. Whence did he learn (to do this)? From our father Abraham.
Aaron and All the Elders of Israel Came to Eat Bread
Mekhilta Tractate Amalek 3:45