Komentarz do Powtórzonego Prawa 13:1

את כל הדבר EVERYTHING [I COMMAND YOU] — light precepts as well as grave ones (Sifrei Devarim 82:1),

לא תוסף עליו, for you might add something which is despicable in the eyes of the Lord. If, perchance, you would add new ways of the serving the Lord, you have no assurance that such innovation is acceptable to G’d, for instance the burning of one’s children in G’d’s honour.

Five totafos within the tefilin, etc. See my explanation above in Parshas Va’eschanan (4:2). Rashi’s statement, “five species with the lulav,” refers to where he binds them so that the five species are all bound together — For [only] then does he transgress. But if he did not bind them together, he does not transgress. Even according to the viewpoint that a lulav does not need binding, it is nevertheless understood that if he binds them together he transgresses the prohibition of, “Do not add.”