Aharon rejoiced in the fact that Hasehm designated Moshe to be the prophet and leader to take the Jews out of Egypt. Even though Aharon himself was the firstborn, and had been a navi and a leader longer than Moshe had. For a reward, Aharon and his descendents recieved the Urim veTumim. The Ran explores why this is a fitting reward, and why in general does Hashem reward “measure for measure”.

The text is available with footnotes at https://www.aishdas.org/kloiz/Derashos-haRan-derashah-3.pdf (starting at ¶ 15).

Or on Sefaria, beginning at https://www.sefaria.org/Derashot_HaRan.3.14

Derashos haRan is an explanation of the fundamentals of the Torah as explicated from texts in the Chumash. In the third derashah, the Ran discusses Moshe, Aharon, the nature of nevu’ah, Moshe’s special kind of nevu’ah,, the urim vetumim, and the first mitzvos given to the Jewish People — counting months from Nissan and of Pesach.