Genesis 49:3

49:3 Reuben, you are my firstborn,

my might and the beginning of my strength,

outstanding in dignity, outstanding in power.

Psalms 105:36

105:36 He struck down all the firstborn in their land,

the firstfruits of their reproductive power. ¹ 

[105:36]  ¹ tn Heb “the beginning of all their strength,” that is, reproductive power (see Ps 78:51).