Acts 7:8

And God gave Abraham a covenant [a formal agreement to be strictly observed] of [which] circumcision [was the sign]; and so [under these circumstances] Abraham became the father of Isaac, and circumcised him on the eighth day; and Isaac [became the father] of Jacob, and Jacob [became the father] of the twelve patriarchs. [Gen 17:10-14; 21:2-4; 25:26; 29:31-35; 30:1-24; 35:16-26]