Mischna zu Schemot 20:21

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).