Sforno on Exodus 20:6/ספורנו על שמות כ׳:ו׳

1ועושה חסד לאלפים, on other occasions the reason why retribution is so long delayed is due to the good deeds of someone’s forbears which I have sworn to requite by extending such patience in tolerating the errors of their offspring. [some of our sages attribute the continued existence of the Ishmaelites and their enmity to the Jews to the merit their founding father acquired when voluntarily submitting to circumcision at the tender age of 13. Ed.]