Rashi on Deuteronomy 4:2/רש"י על דברים ד׳:ב׳

1לא תספו YE SHALL NOT ADD — 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 And to place five fringes on one’s garment. Thus, too, must we explain the following words ולא תגרעו, Ye shall not diminish [from it]" (Sifrei Devarim 82:4).