Sforno on Genesis 21:12/ספורנו על בראשית כ״א:י״ב

1אל ירע בעיניך על הנער ועל אמתך. כל אשר תאמר אליך שרה, do not be angry regarding anything that Sarah says to you concerning the lad and the maid, i.e. to expel them while they wear proof that they are slaves. Avraham’s having placed Ishmael’s on Hagar’s shoulder was a clear sign that Hagar was a slave woman who was charged with carrying Ishmael (verse 14)

3כי ביצחק יקרא לך זרע, for your descendants will be known through Yitzchok, and Ishmael and his sons will be slaves to him. Ishmael will not be known historically for his being a son of Avraham.