זוהר שמות — דף קפה ע”א

A parable about the guard who entered the king’s banquet: the king prepared a feast for himself and his son, and gave the guard a portion on the side so that he would not disturb the joy. Similarly, on Rosh Hashanah, the Holy One Blessed Be He prepares a supernal feast for Israel, but the other side receives a lateral portion in the form of seventy-two goats. And by virtue of this, the joy of Israel at the full festival is preserved.