John Gill Commentary Genesis 15:4

"And, behold, the word of Jehovah came unto him, saying, This man shall not be thine heir; But he that shall come forth out of thine own bowels shall be thine heir." — Genesis 15:4 (ASV)

And behold, the word of the Lord [came] to him
Either having disappeared, and then came a second time, or he again spoke to him:

saying, this shall not be your heir ;
this Eliezer, this servant of yours, as you have made him, or have intended to make him, giving up all hopes of having issue by Sarai:

but he that shall come forth out of your own bowels shall be your
heir ;
that is, one shall inherit all you have, that shall be begotten by you; an own son of Abram's, and not a servant born in his house; one that should spring out of his own loins: the Vulgate Latin version renders it, "out of thy womb", that is, out of his wife's, which was his; the phrase designs a genuine and legitimate son of his, who would be legally his heir.