Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.4.3

6.4.4.3

atha rāsabham | sthiro bhava vīḍvaṅga āśurbhava vājyarvanniti sthiraśca bhava vīḍvaṅgaścāśuśca bhava vājī cārvannityetatpṛthurbhava suṣadastvamagneḥ purīṣavāhaṇa iti pṛthurbhava suśīmastvamagneḥ paśavyavāhana ityetattadrāsabhe vīryaṃ dadhāti

Then (he addresses) the ass, with (Vâg. S. XI, 44), 'Steadfast be thou, firm-limbed, and a swift racer be thou, O steed!'--that is, 'be thou steadfast, and firm-limbed, and swift, and a racer, O steed!'--'Ample be thou, and well to sit upon, thou, the bearer of Agni's supply!'--that is, 'be thou ample (broad), well to rest upon, thou, Agni's provender-bearer!' He thereby lays vigour into the ass.