יֵשֵׁ֤ב בָּדָד֙ וְיִדֹּ֔ם כִּ֥י נָטַ֖ל עָלָֽיו׃
ער זאָל זיצן אַלײן און שװײַגן, װען מע האָט אַרױפֿגעלײגט אױף אים;
Rambam
From where [is there proof that] that even [when there is only] one [person] sitting, etc.: In the first chapter of Berakhot 6a, it states in this language, "And from where is it derived that one who sits and engages in Torah [study], the Divine Presence is with him? As it is stated: 'In every place where I cause My Name to be mentioned, I will come to you and bless you' (Exodus 20:21); Since even one, was it necessary [to say] two? [With] two, their words are written in the book of remembrance; with one, his words are not written in a book of remembrance. And since even two, was it necessary [to say] three? Lest you say that judgment is merely peace, and the Divine Presence does not come. [Instead,] we understand that sitting in judgment is also Torah. And since even three, was it necessary [to say] ten? [With] ten, the Divine Presence precedes [them] and comes; [with] three, not until they sit." And the explanation of "he is silent (vayidom)" as being from the hidden speech is from "a still (demamah) small voice" (I Kings 19:12). And from this [idea] did the [Onkelos' Aramaic] translation explain, "And Aharon was silent" (Leviticus 10:3) [as] "And Ahaon praised." And his proof that he is like one that observed the entire Torah completely is from his saying, "since he takes it on himself" (Lamentations 3:28) - it is as if the giving of the entire Torah was only for his sake.
English Explanation of Pirkei Avot
R. Hananiah ben Teradion said: if two sit together and there are no words of Torah [spoken] between them, then this is a session of scorners, as it is said: “nor sat he in the seat of the scornful…[rather, the teaching of the Lord is his delight]” (Psalms 1:1); but if two sit together and there are words of Torah [spoken] between them, then the Shekhinah abides among them, as it is said: “then they that feared the Lord spoke one with another; and the Lord hearkened and heard, and a book of remembrance was written before Him, for them that feared the Lord and that thought upon His name” (Malachi 3:16). Now I have no [scriptural proof for the presence of the Shekhinah] except [among] two, how [do we know] that even one who sits and studies Torah the Holy One, blessed be He, fixes his reward? As it is said: “though he sit alone and [meditate] in stillness, yet he takes [a reward] unto himself” (Lamentations 3:28). From the verse in Psalms used as a prooftext we can see that the opposite of a gathering of scorners (synonymous with sinners) is a gathering for the study of Torah. Therefore any gathering in which Torah study is completely absent is considered a gathering of scorners. Furthermore, the statement is that there must be between them words of Torah. It is not sufficient that each studies or meditates upon Torah on his own. The words of Torah should be shared with others. However, even two people who gather to study Torah cause the Shekhinah, the Divine Presence, to abide amongst them. This is learned from the verse in Malachi, which implies that when the group of God-fearers gathered, God hearkened to their words, for He dwelled amongst them. Finally, even one who studies on his own, receives a reward for such study.
Rabbeinu Yonah
As it is said (Lamentations 3:28): “He sits alone and is silent, since he takes [a reward] for it”: As one who is silent and thinks about Torah is as if he takes on the yoke of Torah in oral meditation. But Rabbi Meir HaLevi follows the textual variant, "From where [is there proof that] that even [when there is only] one [person studying Torah], the verse equates it for him as if he fulfilled the whole entire Torah, as it is said (Lamentations 3:28): 'He sits alone and is silent, since he takes [a reward] for it.'" And how is it implied that the expression "and he is silent" is [about] speech? [From] "since he takes" [which refers] to the yoke of speech - as he preaches in front of [the people].