פירוש על עבודה זרה 4:1

רבי ישמעאל אומר. זו בצד זו – and all the more so, one on top of two which is the essence of Merkulis/Mercury/Hermes. [Merkulis is the name of idolatry that is called in their languages, Merkuris.]

Rabbi Ishmael says: if three stones are lying side by side next to a merculis, they are prohibited; if there are two they are permitted.
The sages say: if [the stones] are seen to be connected with it they are prohibited, but if they do not appear to be connected with it they are permitted.
In Sanhedrin 7:6 we learned that one who throws stones at a statue of merculis (the Roman god, mercury) is guilty of worshipping the idol, for that his how merculis is typically worshipped. Our mishnah deals with the status of stones found next to the merculis statue.
According to Rabbi Ishmael, if three stones were found next to the merculis statue, we can assume that they were used in worshipping the statue, and they are therefore prohibited. If there were only two, then we cannot assume that they were placed there for such a purpose and they are permitted.
According to the Sages the issue is not the number of stones but rather their proximity to the statue. Those found next to the idol are prohibited and those found further away are permitted. Even if three are found further away, we can assume that they were not used in worship.

בצד מרקוליס – at the side four cubits of Merkulis, meaning to say, that the stones are four cubits distant from Merkulis and it is known and that they did not fall from him, and on this, Rabbi Yishmael states that three stones are prohibited for Merkulis is not less than three stones and they make a small Merkulis at the side of a large Merkulis and a small one made at the side of a large one, they worship it with anything and we should not suspect that there will be one [stone] on top of two [stones]. But the Rabbis hold that we do not make a small Merkulis at the side of a large Merkulis. Therefore, it appears with it, meaning to say, next to it, for one can say that it fell from it. Whether two [stones] or three [stones], it is forbidden. If it is not seen with it, it is permitted. And the Halakha is according to the Sages.