Josh 1:12

“Let us look closely at what sign those two and a half tribes hold who receive the land of inheritance through Moses, and what sign the remainder of the nine and a half tribes hold who receive the promise of the holy land through Jesus [Joshua].First of all, I think it is impossible for it to have happened accidentally that those who receive a portion through Moses were all firstborn. For Reuben was the firstborn of Leah; Gad, the firstborn of Zilpah; and Manasseh, the firstborn of Asenath the Egyptian whom Joseph married, the daughter of Potiphar the priest of Heliopolis. For myself, I can never be persuaded that it was by chance that the firstborn were the only ones whose inheritance was determined through Moses. Rather, I believe that in these things the design of two groups of people was already foreshadowed at that time: One would seem to be the firstborn according to the order of nature; the other, the people who would receive the blessing of their inheritance through faith and grace.”