Sforno on Genesis 15:9/ספורנו על בראשית ט״ו:ט׳

1קחה לי עגלה, take a heifer for Me, in order to enter into a covenant concerning this assurance. Converting G’d’s promise into what is known as a “covenant,” is equivalent to G’d reinforcing His promise by an oath, something which is unconditional and irrevocable. (Rosh Hashanah 18) Moses himself confirmed this interpretation when he said (Deut 9,5) “not due to your righteousness or the uprightness of your heart, but due to the wickedness of these nations does G’d dispossess them on your account; and in order to keep His promise which He had sworn to your forefathers Avraham, Yitzchok and Yaakov