Midrasz do Pirke Awot 3:22

Rabbi Phineas said: (After) forty years Moses wished to say to Israel: Do ye remember that which ye said in the wilderness—"Is the Lord among us, or not?" (Ex. 17:7). But Moses said: If I speak thus to Israel, behold I will put them to shame, and whosoever puts (his fellow) to shame will have no portion in the world to come.