Sifrei Devarim
"and so that he not turn aside from the mitzvah": even a "slight" mitzvah of a prophet. And thus is it written (I Samuel 13:13) "And Samuel said to Saul: You have acted foolishly. You have not kept the mitzvah, etc." "and so that he not turn aside from the mitzvah, right or left": even if he (seems to be) told that right is left and left is right.