זוהר בראשית — דף ריח ע”ב

The tribes of God are all righteous and all holy, bearing witness to one another's righteousness. When Jacob stood to bless his sons, he placed them facing the entire Merkava (Divine Chariot). The seventy souls who went down to Egypt correspond to the seventy nations. The blessing of Reuben opens the sequence of blessings for all the tribes.