The history of Circumcision - Genesis 17
God sealed his convanant with Abram by as he changed his name to Abraham.
He instituted the convanant of circumcision as a way to keep his convanant with Abraham till eternity.
God changed Sarai's name to Sarah and promised to give her a son that will be called Isaac.
Abraham got circumcised at the age of 99, so did Ishmael and all of his slaves on the same day he did.