Genesis 15-17:27

Abram is childless, God tells him his seed will be as the stars in the sky, numberless.

    Abram believes God but he asks how he will know that it is true.  God tells him to prepare a sacrifice, then gives him a dream, then smokes the sacrifice to seal the deal. Wow.........

Then Sarai wants to help God out and presents Hagar to be a surrogate.  Then Hagar gets pregnant and Sarai hates her.  Isn't that the way we are!  Don't be trying to help God out!  He has a plan and will orchestrate things just fine!

Anyway, Ishmael is born and when God tells Abraham when he is 100 years old he and Sarah will have a child when she is 90, he falls on the ground laughing.  Don't be ridiculous God - well, God can do anything He wants, anytime.  He argues with God saying Ishmael will be the son and God says, nope, Sarah will have a son and call his name Isaac.

Notice that Abram and Sarai got the name change here.

Also, circumcision began at this time.