Anushasana Parva 149.27

Dridheyu, Riteyu, illustrious Parivyadha, Ekata, Dvita, and Trita, the last three gifted with solar effulgence, and Atri's son of righteous soul, viz., the Rishi Sarasvata, these seven who had acted as Ritvijas in the great sacrifice of Varuna, have taken up their abodes in the Western quarter.