It was Isaac. God sent Ismail and Hagar away. The birth of Ismail represents lack of faith in God or trying to play God and the consequences of trying to play God. Abraham and Sarah didn’t trust God at His word and tried to figure how they would have descendants as numerous as the stars in the sky, they came up with a plan to use Hagar to birth their children but that wasn’t what God told them was going to happen. Isaac is also an Old Testament type for Jesus. Abraham would have been over a 100 and Isaac would have been a teenager hence why the Bible notes that Isaac carries the wood not Abraham, Abraham was probably too old. Isaac was a willing victim and trusts he father with his life he could have over taken Abraham, and in 5000 years God is going to send His only beloved son up that same mountain carrying wood and being a willing victim for the sins of the world. Had God not sent Ismail away, I am sure Abraham would have called him instead of Isaac because Isaac is was the beloved son.