“And you shall not take away from them” (Devarim 13,1)
It is a negative commandment not to take away anything from that which our perfect Torah has obligated us. Concerning this it says “and you shall not take away from them”. When does this prohibition apply? For example, Chazal say that if the blood of an offering which must be given in one application gets mixed with the blood of an offering which must be given with four applications, according to Rabbi Yehoshua, if he gives them all with one application he transgresses the prohibition of “You shall not take away”. We know from here that this and anything similar to it, is a prohibition of “You shall not take away”.
Amongst the roots of the commandment, is as we explained in the commandment of “You shall not add”, which pre-empts it, and the rest of its details are similar.