'Moses commanded us a law, the inheritance of the congregation of Jacob'. I will explain this according to the commentators, of blessed memory. Since our holy fathers observed the Torah and took its yoke upon themselves, as we learned at the end of Tractate Kiddushin: Abraham our father fulfilled the Torah in its entirety, even before it was given, as it says 'Because Abraham listened to My voice, and he observed my commandments, laws, and teachings'. If so, we too are obligated to observe it, the reason being 'forsake not the teaching of thy mother', and cannot claim to having been forced [as in] the statement 'He held the mountain over their heads like a barrel'.
This is why 'Moses commanded us a law' was said. Despite the claim of having been forced, since 'He held the mountain like a barrel', we are, in any case, obligated to observe it for the reason that it is 'an inheritance of the congregation of Jacob' – the chosen among our fathers. Therefore we undertook the obligation to it [law] on the basis of 'forsake not the teaching of thy mother' so that nothing remains of the statement claim, simple to understand.