Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.2.3

6.3.2.3

atha rāsabham | yuñjāthāṃ rāsabhaṃ yuvamityadhvaryuṃ caitadyajamānaṃ cāhāsminyāme vṛṣaṇvasū ityasminkarmaṇi vṛṣaṇvasū ityetadagnim bharantamasmayumityagnim bharantamasmatpreṣitamityetattadrāsabhe vīryaṃ dadhāti

Then the ass, with (Vâg. S. XI, 13), 'Yoke ye two the ass,' he says this to the Adhvaryu and the Sacrificer;--'upon this course, ye showerers of wealth!'--that is, 'upon this performance, ye showerers of wealth;'--'him, bearing Agni, and helpful unto us;'--that is, 'him, bearing Agni, and urged forward by us:' he thereby lays vigour into the ass.