Genesis 15:4 Then the word of the Lord came to him "This man will not be your heir, but a son coming from your own body will be your heir".
God talking To Abram, telling him he would have his own son, his own flesh and blood as an heir.
Abram believed him, but then took matters into his own hands. He didn't inquire or ask of God. He didn't wait for God's timing.
When Sarai, his wife, suggested he sleep with her maid servant, Hagar, to have her bear a son, he readily agreed to the plan (Genesis 16:1-4) Hagar did conceive and Ishmael was born.
Interestingly, Hagar runs away before giving birth as Sarai was mistreating her because she was jealous of Hagar's pregnancy. The angel of the Lord tells her to go back to Sarai and adds "I will so increase your descendants that they will be too numerous to count," which is indeed what happens in that Ishmael has 12 sons and the religion Islam is linked to him.
Ishmael was born when Abram was 86 yeas old. When Abram is 99 years old, God appears to him and tells him he and Sarai will have a child together. He reconfirms the covenant he had made with Abram of his descendants being numerous, changes Abram's name to Abraham, Sarai's name to Sarah, gives circumcision as part of the covenant.
Abraham laughs at God about them having a son, who God says they will call Isaac. God says this time new year Sarah will bear a son.
He clearly laid out his plan, yet Abraham had trouble believing it. God is capable of doing the impossible. We must seek his will, not take matters into our own hands and we must wait for his perfect timing. How different things might have been if Sarai and Abram waited, inquired of God, and waited for him to give the heir he promised born of both of them.