Or HaChaim on Leviticus 26:45/אור החיים על ויקרא כ״ו:מ״ה

1 וזכרתי להם כרית ראשונים, "And I will remember for their sakes the covenant with the ancestors, etc." Torat Kohanim asks: "whence do we know that G'd concluded a covenant with the tribes?" They quote our verse as the source. Perhaps we should understand this in the way Jeremiah quotes G'd as remembering the time after the crossing of the sea of reeds when the Israelites willingly followed G'd into the desert, etc. (compare Jeremiah 2,2). G'd goes on record here that He will remember such deeds by our forefathers to their descendants in due course. The word ראשונים refers to the first ever generation of the Jews as a people G'd concluded a covenant with.