The LORD appeared to Abraham by the oaks of Mamre as three visitors came to his tent. One announced that Sarah would have a son within the year, and the LORD answered her laughter by asking whether anything was too difficult for Him (Genesis 18:1-15).
1Then the LORD appeared to Abraham by the Oaks of Mamre in the heat of the day, while he was sitting at the entrance of his tent. 2And Abraham looked up and saw three men standing nearby. When he saw them, he ran from the entrance of his tent to meet them and bowed low to the ground. 3“My lord,” said Abraham, “if I have found favor in your sight, please do not pass your servant by.
Paul quotes God's promise that He would return at the appointed time and Sarah would have a son.
For this is what the promise stated: “At the appointed time I will return, and Sarah will have a son.”
Genesis 18:10-14 promises that Sarah will have a son. Romans 9:9 repeats the promise while speaking about God's word to Abraham.