מוסר אויף הושע 4:6

And [(by neglecting Torah study)] he especially injures himself, as Chazal have said: "Whoever forgets one item of his learning brings bout exile to his sons, as it is written (Hoshea 4:6): 'And you have forgotten the Torah of your G-d — I, too, will forget your sons.'"

And as it is stated in the first chapter of Megillah (9a), the story of Ptolemy who gathered seventy-two elders and so on, and they changed the Torah. There, it was from Heaven that they agreed with him, and it was as though it were inspired by the Holy Spirit. And it also says there that God placed it in the heart of each one, and so on, and this was according to the word [of God]. Furthermore, there, they did not change anything from its meaning, but only changed the wording so it would not lead to heresy. And as we learned, this is indeed the plain meaning of the verse. But to say that an exemption is an obligation, or the opposite, would be as if denying the Torah of Moses. What difference does it make whether it is one word or the entire Torah? As it is stated in the third chapter of Yoma (38b), Rav said: ‘Anyone who forgets one thing from their learning causes exile for their children,’ as it is written (Hosea 4:6), ‘Because you have forgotten the Torah of your God, I will also forget your children.’ Thus, even one word from the Torah is considered the Torah of God.