משנה אויף מלאכי 3:16

Rabbi said: which is the straight path that a man should choose for himself? One which is an honor to the person adopting it, and [on account of which] honor [accrues] to him from others. And be careful with a light commandment as with a grave one, for you did know not the reward for the fulfillment of the commandments. Also, reckon the loss [that may be sustained through the fulfillment] of a commandment against the reward [accruing] thereby, and the gain [that may be obtained through the committing] of a transgression against the loss [entailed] thereby. Apply your mind to three things and you will not come into the clutches of sin: Know what there is above you: an eye that sees, an ear that hears, and all your deeds are written in a book.

Rabbi Hanina, the vice-high priest said: pray for the welfare of the government, for were it not for the fear it inspires, every man would swallow his neighbor alive. 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).

Rabbi Halafta of Kefar Hanania said: when ten sit together and occupy themselves with Torah, the Shechinah abides among them, as it is said: “God stands in the congregation of God” (Psalm 82:1). How do we know that the same is true even of five? As it is said: “This band of His He has established on earth” (Amos 9:6). How do we know that the same is true even of three? As it is said: “In the midst of the judges He judges” (Psalm 82:1) How do we know that the same is true even of two? As it is said: “Then they that fear the Lord spoke one with another, and the Lord hearkened, and heard” (Malachi 3:16). How do we know that the same is true even of one? As it is said: “In every place where I cause my name to be mentioned I will come unto you and bless you” (Exodus 20:21).