Exode 20:6

Doing kindness. That a person does, [so God will] repay a reward to the thousandth generation. When we find a good characteristic, it is greater than even one evil deed in five hundred, for this one is to four generations, and that one is to the thousandth.

but keeping mercy and goodness for thousands of generations of the righteous who love Me, and who keep My commandments and My laws.

AND HE SHOWETH MERCY UNTO THE THOUSANDTH GENERATION OF THEM THAT LOVE ME AND KEEP MY COMMANDMENTS. It appears from the sense of the verse that this Divine assurance is with respect to the subject-matter of the commandments that He mentioned. He is thus saying that to those that love Him, He will show mercy to their thousandth generation. These are the ones who sacrifice their lives for Him, for they are the ones who acknowledge only the Glorious Name and His G-dship and deny all strange gods, refusing to worship them even if they are in mortal danger. They are called “the lovers [of G-d],” for this is the kind of love that we have been obligated to observe even at the sacrifice of life, just as He has said, And thou shalt love the Eternal thy G-d with all thy heart, and with all thy soul, and with all thy might, meaning that you should give your very life because of your love of Him, that you should not alter Him for another god, nor join Him together with a strange god. It is for this reason that [the prophet] said of Abraham, the seed of Abraham my friend, since he risked his life in order not to worship the idols in Ur of the Chaldees. The rest of the righteous are called those that keep My commandments.
Now many scholars have explained that “His lovers” are those who worship Him without the intention of receiving a reward, just as our Sages have mentioned. But I have found in the Mechilta that it is said: “Of them that love Me. This refers to Abraham and those like him. And those that keep My commandments. This refers to the prophets and the elders. Rabbi Nathan says that the verse, of them that love Me and keep My commandments, refers to those who dwell in the Land of Israel and give their lives for the commandments. ‘Why are you being led out to be executed?’ ‘Because I have circumcised my son.’ ‘Why are you being led out to be burned?’ ‘Because I read the Torah.’ ‘Why are you being led out to be hanged?’ ‘Because I ate the unleavened bread.’ ‘Why are you being lashed with the whip?’ ‘Because I took the lulav.’ And it says, Those with which I was wounded in the house of my friends. These are the wounds which have caused me to become beloved of My Father in heaven.” Thus Rabbi Nathan explained that the love [of G-d, which is referred to in the verse before us], meant the sacrifice of life for the sake of the commandments. Now the verse here certainly refers to idolatry, for it is with reference to it that we are obligated at all times forever to suffer death rather than transgress [the law]. But [Rabbi Nathan] broadened the matter to include all the commandments, [such as circumcision, the study of Torah, the eating of unleavened bread on Passover, the taking of the lulav on Succoth — as mentioned above] — because in the time of religious persecutions, we are obligated to suffer death for any of the commandments [rather than transgress them], as derived from the other verse, And ye shall not profane My holy Name. And it would also be incorrect to say of the first Sage [in the above Mechilta] — i.e., who said that [those who love Me] refers to Abraham, while [those who keep My commandments] refers to the prophets — that he is of the opinion that the prophets kept the commandments with the intention that they receive a reward! [Thus the explanation of “the many scholars” mentioned above is refuted by the Mechilta.] However, there is a secret in this [Mechilta]: Abraham risked his life in love — something like it is written, mercy to Abraham — and the rest of the prophets in g’vurah (might). Understand this.

ועושה חסד לאלפים, to the third and fourth generation, fifth, tenth, hundredth and thousandth. The term אלפים mentioned here is equivalent to the אלף דור, 1000 generations mentioned in the parallel version of the Ten Commandments in Deuteronomy 7,9. The two verses need not be a contradiction to one another according to the simple meaning of the text. Here the Torah mentions children, grandchildren, and great grandchildren. All “children” are part of the same generation. All grandchildren are the same but second generation. The children after the 1000th generation are referred to as אלפים, or “the plural mode of one thousand.” In Deuteronomy, where children and grandchildren have not even been mentioned, and no mention is made of third and fourth generations, only the final generation to whom G’d extends such love is mentioned. Hence it is natural that this “final” generation qualifying for this loving care by G’d is called אלף דור, i.e. this generation being the children of the 1000th generation. In other words, both verses speak about the same generation being the last generation enjoying this love of G’d for something their originators so long ago did which pleased G’d.

ועושה 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.]