Midrasch zu Bamidbar 4:1

1) "And He called to Moses and the L–rd spoke to him, etc." "to him" — to exclude Aaron. R. Yehudah b. Betheira said: Thirteen dibroth (accompanied by a command) were stated in the Torah to Moses and Aaron, and, corresponding to them, thirteen limitations, to teach us that they were not spoken to Aaron, but to Moses, to tell them to Aaron. (The dibroth: 1) [Shemoth 6:13]; 2) [Shemoth 7:8]; 3) [Shemoth 9:8]; 4) [Shemoth 12:1]; 5) [Shemoth 12:43]; 6) [Vayikra 11:1]; 7) [Vayikra 13:1]; 8) [Vayikra 14:33]; 9 [Vayikra 15:1]; 10 [Bamidbar 2:1]; 11 [Bamidbar 4:1]; 12 [Bamidbar 4:18]; 13) [Bamidbar 19:2].)