Sanhedrin, Chapter Eleven, Mishnah Three
Mishnah three continues to discuss the rebellious elder.
1) There is greater stringency in respect to the teachings of the scribes than in respect to the torah: [thus,] if [a rebellious elder] says, there is no commandment of tefillin, so that a biblical law may be transgressed, he is exempt.
a) [But if he rules that the tefillin must contain] five compartments, thus adding to the words of the scribes, he is liable.
This mishnah states that a rebellious elder who teaches against a Rabbinic teaching, one not mentioned explicitly in the Torah, is dealt with more severely than one who teaches against the explicit words of the Torah. An elder who states that there is no commandment of tefillin is not to be executed, since the commandment of tefillin is stated explicitly in the Torah (Exodus 13:9, 16, Deuteronomy 6:8, 11:18). Anyone who hears this elder teach that there is no tefillin will be able to check the Torah and realize that the elders teaching is false. However, one who states that the tefillin must be made of five boxes, instead of the four prescribed by the Rabbis, is liable for the death penalty. Since the Torah does not state how many boxes are to be in the tefillin, this is a teaching that if we didnt know from the Rabbis we would not know at all. Therefore, an elder who teaches against this teaching is more of a threat than the elder who teaches against something stated specifically in the Torah.
Note: The tefillin worn on ones head contain four boxes but the one worn on the arm contains only one box. From the outside each is painted black and therefore looks like one box.