Śatapatha Brāhmaṇa ("of a hundred paths") 9.4.2.16

9.4.2.16

yadveva vāruṇīṃ juhoti | prajāpatervisrastādvīryamudakrāmattaṃ yatra devāḥ samaskurvaṃstadasminnetayā vīryamadadhustathaivāsminnayametaddadhāti vāruṇya 'rcā kṣatraṃ vai varuṇo vīryaṃ vai kṣatraṃ vīryeṇaivāsminnetadvīryaṃ dadhāti

And, again, as to why he offers one relating to Varuna. When Pragâpati was dismembered, his strength departed from him. When the gods restored him they, by this (oblation), bestowed strength upon him; and in like manner does this (Sacrificer) bestow it upon him. With a verse addressed to Varuna (he offers),--Varuna is the ruling power, and ruling power means strength: it is thus by strength that he bestows strength upon him.