Sforno on Genesis 15:4/ספורנו על בראשית ט״ו:ד׳

1לא יירשך זה כי אם אשר יצא ממעיך, for the son would succeed taking over Avram’s leadership role amongst large number of followers while his father was still alive. This is the true meaning of Genesis 25, 5-6 ויתן אברהם את כל אשר לו ליצחק בנו.... בעודנו חי,” Avraham handed over all that was his to his son Yitzchok while he was still alive.” This is followed by the statement that Avraham sent away the sons of the concubines from the presence of his son Yitzchok.