Rashi on Deuteronomy 13:1/רש"י על דברים י״ג:א׳

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

2תשמרו לעשות OBSERVE TO DO — This statement is intended to attach also a prohibition (לאו) to the positive commands mentioned in this section (Sifrei Devarim 82:2), for wherever השמר is used it is a term that implies a prohibition, but the difference between this and an ordinary prohibition is that flagellation is not inflicted for infringing a prohibition resulting from the use of the term השמר in connection with a positive command.

3לא תסף עליו THOU SHALT NOT ADD THERETO — as, for instance, to place five chapters in the Tephillin, to employ five species of fruit and plants in the fulfilment of the command of Lulab, to recite four instead of three blessings in the Benedictions of the priests (Sifrei Devarim 82:4; cf. Rashi on Deuteronomy 4:2).