Halakha Ulangan 4:2: Mishneh Torah, Transmission, Mishneh Torah, Prayer and the Priestly Blessing & Shulchan Arukh, Orach Chayim

but the commandment, which is the oral explanation of the Torah, he did not reduce to writing, but he charged the Elders and Joshua and the rest of all Israel concerning its observance, even as it is said: "All the word which I command you, that shall ye observe to do" (Deut. 13.1.); therefore, is this word of the commandment called, Oral Torah.

In no place may the priests add, either softly or aloud, a blessing to the three verses of the Benediction, such as, "The Lord, the God of your fathers, make you a thousand times as many more as you are", or other verses of a similar tenor, for it is said, "Thou shalt not add to it" (Deuteronomy 13:1). Every priest proceeding to the platform to pronounce the Benediction, as soon as he starts, says, "May it be Thy Will, O Lord our God, that this Benediction which Thou hast commanded us to bless therewith Thy people Israel shall be a perfect blessing. May there be in it no stumbling nor perverseness from now and for evermore". Before the priest turns his face towards the people to bless them, he recites the blessing, "Blessed art Thou, O Lord our God, King of the Universe, who hast sanctified us with the sanctification of Aaron and commanded us to bless Thy people Israel with love". He then turns his face to the congregation and begins to bless them. And when he turns his face away from the congregation, after he had concluded the Benediction, he says "We have done that which Thou hast decreed unto us. Do Thou fulfill unto us that which Thou hast promised unto us. Do Thou look down from Thy holy habitation, from Heaven, and bless Thy People Israel".

One should not add another species to the four species due to the prohibition against adding.