In the Gemara Megilah is brought this verse and explanation in Deuteronomy 12, '' These are the commandments that God has set before you this day..'' ''From here we learn that no prophet can add anything new.'' אין נביא רשאי לחדש דבר מעתה Later I saw brought from the Gra the verse ''You have seen that every man who was joined to Baal Peor [an idol], God destroyed,'' that the worship of Baal Peor was to throw feces at it, and some people had done so thinking that it was a mitzvah. From here the Gra also shows that one not ought to go around looking for mitzvot to do. And in the Sidur haGra is brought a statement from Reb Chaim of Veloshin, ''It is better to sit in one's room and twiddle one's thumbs, rather that to go around looking for mitzvot to do.''
In Deuteronomy 4 it says "Thou shalt not add nor subtract from these commandments" which pretty much sums up this subject.
