Variantly: "you shall not add to it": Whence is it derived that additions are not to be made to a lulav or to tzitzith? From "you shall not add to it." And whence is it derived that nothing is to be subtracted from them? From "and you shall not detract from it."
And whence is it derived that if a Cohein began the priestly blessing, he should not say: Since I have begun to bless, I shall say (Devarim 1:11) "May the L-rd, the G-d of your fathers, add to you, etc." From (Ibid. 13:1) "All the thing (davar), etc." — Even speech (["davar" can also be understood as "speech"]) shall not be added to it.