Question
How do we know that nothing will be added to the Torah?
Answer
It's explicit in the Torah that WE can't add to or subtract from it (Deut.4:2, 13:1). The idea that G-d won't add to the Torah is deducible from the Torah, but isn't explicit. The Talmud derives from "These are the commandments" (e.g., Lev.27:34) the fact that no prophet can add to the Torah; see Shab.104a. The Talmud also derives from "It is not in heaven" (Deut.30:12) the fact the Torah is now in human hands, which implies that it won't be added to.