Rashi on Deuteronomy 11:19/רש"י על דברים י״א:י״ט

1לדבר בם [AND YE SHALL TEACH THEM UNTO YOUR CHILDREN,] TO SPEAK OF THEM — From the moment when your son knows how to speak, teach him the text (Deuteronomy 33:4) “Moses commanded us the Torah as a possession of the congregation of Jacob” — so that this should be the means of teaching him to speak (Sukkah 42a). From this they (the Rabbis) derived their teaching: When the babe begins to speak, his father should speak with him in the Holy Tongue, and should instruct him in the Torah. If he does not do this, it is as though he buries him, as it is said here, “And ye shall teach them unto your children to speak of them, etc.”,