Numbers 3:9-10
9 And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him out of the children of Israel.
10 And thou shalt appoint Aaron and his sons, and they shall wait on their priest's office: and the stranger that cometh nigh shall be put to death. |
Maybe you are thinking of a Midrash?
Why Moses Did Not Become A Priest
A midrash tells us that during the seven days when Moses was at the burning bush each day he pleaded with God to send someone else (which we know from the Torah text). In the end of the midrash, God informs Moses that because of his unwillingness to take on the task during those seven days he will not be permitted to ascend to the priesthood. Rather, it is Aaron and his descendants who will become the priests. However, for seven days, when the Mishkan is dedicated, Moses will be allowed to perform the priestly functions, but not after that.
Moses' reaction to what might be perceived as a punishment is to rejoice over the good fortune of his elder brother Aaron. After all, Midrash tells us, one reason why Moses is reluctant to take on the leadership role is because he is afraid that Aaron will be jealous that his younger brother is the leader of the people. However, God informs him that Aaron will rejoice at seeing Moses and hearing that he is to lead the mission to Pharaoh, and indeed he does. For this Aaron is rewarded; let “that same heart that rejoiced in the greatness of his brother [have] precious stones (the priestly breastplate) set upon it.”