הלכה על סוטה 39:15

And this commandment is practiced in every place and at all times by males, but not by females, since it is stated (Deutoronomy 1:19), "your sons" - and they, may their memory be blessed, expounded (Kiddushin 29b), "But not your daughters." And so [too,] a woman is not obligated to teach her son, as anyone who [does not have] the obligation to study, [does not have] the obligation to teach. But even though she is not commanded by Torah writ, nonetheless it is fitting for every woman to make efforts that her sons should not be ignoramuses, and she has good reward [for this] in general (based on Sotah 21a). And there is also reward for a woman who studies Torah. But nonetheless, the Sages commanded (Sotah 20a) that a man not teach his daughter Torah, because the mind of women is weak, and they turn words of Torah into words of nonsense through the poverty of their minds. And one who transgresses it and does not teach his son Torah - until he knows how to read in the Scroll of the Torah and understand the simple understanding of the verses - has violated this positive commandment. And anyone who has the wherewithal in his hand to study in any way is included in this positive commandment. And if he does not fulfill it, his punishment is very great, as this commandment is the mother of them all.