Then the word of the LORD came to Abram, saying, “This one will not be your heir, but one who comes from your own body will be your heir.”
The word of the LORD answers that this servant will not be Abram's heir; the heir will come from Abram's own body.
This verse directly corrects Abram's assumption and narrows the promise.
1After these events, the word of the LORD came to Abram in a vision: 2But Abram replied, “O Lord GOD, what can You give me, since I remain childless, and the heir of my house is Eliezer of Damascus?” 3Abram continued, “Behold, You have given me no offspring, so a servant in my household will be my heir.” 4Then the word of the LORD came to Abram, saying, “This one will not be your heir, but one who comes from your own body will be your heir.” 5And the LORD took him outside and said, “Now look to the heavens and count the stars, if you are able.” Then He told him, “So shall your offspring be.” 6Abram believed the LORD, and it was credited to him as righteousness. 7The LORD also told him, “I am the LORD, who brought you out of Ur of the Chaldeans to give you this land to possess.”