Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.4.7

6.4.4.7

tamaśvasyopariṣṭātpragṛhṇāti | praitu vājī kanikradaditi praitu vājī kanikradyamāna ityetannānadadrāsabhaḥ patveti tadaśvasya yajuṣi rāsabhaṃ nirāha tadrāsabhe śucaṃ dadhāti bharannagnim purīṣyam mā pādyāyuṣaḥ pureti bharannagnim paśavya mo asmātkarmaṇaḥ purā pādītyetattadenamaśvena sambharati

In the first place he holds it over the horse, with (Vâg. S. XI, 46), 'Let the racer start forth neighing lustily,'--that is, 'Let the racer start forth neighing repeatedly;'--'the running ass, crying aloud!' He thus mentions the ass in the formula of the horse, and thereby imbues the ass with sorrow;--'bearing Agni Purîshya, may he not perish before his full measure of time!'--that is, 'bearing Agni favourable to cattle, may he (the horse) not perish before (the completion of) this sacred work.' He thereby equips him (Agni) with the horse.