'RChalafta ben Dosa, leader of Kfar Chananiah, says: [When] ten people are sitting [together] and engage in Torah study, the Divine Presence dwells among them, as it is stated: God stands in the Divine congregation.  And from where [do we know] even five? As it is stated: And He has founded His group upon the earth.  And from where [do we know] even three? As it is stated: In the midst of judges He shall judge.  And from where [do we know] even two? As it is stated: Then those who fear Hashem spoke to each other, and Hashem listened and heard, etc.  And from where [do we know] even one? As it is stated: Wherever I cause My Name to be mentioned I shall come to you and bless you.