TT #12 · Parashas Va’eschanan · First Aliyah · Passuk 4:9
Only take utmost care and watch yourselves scrupulously, so that you do not forget the things that your eyes have seen and so that they do not fade from your mind as long as you live. And make them known to your children and to your children’s children.
Torah Temimah points to Kiddushin 30a (teaching one’s grandson is equivalent to receiving the Torah at Sinai).
English: R’ Yehoshua ben Levi said: Anyone who teaches his grandson Torah — Scripture accounts it to him as if he himself received it at Mount Chorev, as it is said “and make them known to your children and to your children’s children,” and immediately afterward it is written “the day that you stood before Hashem your God at Chorev.”
Semichut ha-parshiyos (adjacent clauses) — “vehodatem… u-livnei vanecha” joined to the day of standing at Chorev.
Teaching Torah to one’s grandson is credited as though one stood again at Sinai and received the Torah anew.