Tosefta Horayot
"O House of Israel, bless the Lord, O House of Aaron, bless the Lord, O House of Levi, bless the Lord, Those who fear the Lord, bless the Lord" (Ps. 135:19-20). Consequently, Israel blesses one, the House of Aaron blesses two, the House of Levi blesses three, and those who fear the Lord bless four.