When you teach your son, you teach your son’s son.
The Talmud (AD 200-500) Collection of Jewish rabbinical writings
Seder Nashim, Kiddushin 30a
Paraphrase of "This serves to say to you that whoever teaches his son Torah, the verse ascribes him credit as though he taught him, and his son, and his son’s son, until the end of all generations" (alt. trans. "to him who teaches his son Torah, the Writ ascribes merit as though he had taught him, his son and his son's son until the end of all time!"). This is in turn referenced to Deut. 4:9.
I suspect that this is actually a paraphrase of Seder Nashim, Tractate Kiddushin, 30a. I found a Talmudic reference (36a) to the saying in The World’s Religions After September 11 (2008), by Arvind Sharma [http://bit.ly/2vfVVjc, footnote http://bit.ly/2v7PU7u%5D. However, I think the footnoted reference is a typo and that the saying is actually derived from the reference above [http://bit.ly/2w8d4rs], which says, in part, “whoever teaches his son Torah, the verse [Deuteronomy 4:9] ascribes him credit as though he taught him, and his son, and his son’s son, until the end of all generations.”
And though I haven’t looked at any Hebrew in over 50 years, I believe the original passage from which the English phrase above was translated might be the following:
המלמד את בנו תורה מעלה עליו הכתוב כאילו למדו לו ולבנו ולבן בנו עד סוף כל הדורות
Good catch, Hugh — I believe that’s a fit.