Reference for Chagigah 7:8
אמר מר כל זכורך לרבות את הקטנים והתנן חוץ מחרש שוטה וקטן אמר אביי לא קשיא כאן בקטן שהגיע לחינוך כאן בקטן שלא הגיע לחינוך קטן שהגיע לחינוך דרבנן היא אין ה"נ וקרא אסמכתא בעלמא
The master had said, "All your males" is to include the children [in the obligation]. But doesn't it teach in a Mishnah, Except a deaf man, a mentally ill person and a child? Abaye said, There is no difficulty. One case is regarding a child who has reached the age for education, the other regarding a child who has not reached the age for education. [We learn this from a biblical verse?] A child who has reached the age for education is [only obligated] on a rabbinic level! Indeed this is so. The expression [therefore] is a mere supporting peg [to the rabbinic obligation].