Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.4.8

6.4.4.8

atha rāsabhasya | vṛṣāgniṃ vṛṣaṇam bharanniti vṛṣā vā agnirvṛṣā rāsabhaḥ sa vṛṣā vṛṣāṇam bharatyapāṃ garbhaṃ samudriyamityapāṃ hyeṣa garbhaḥ samudriyastadenaṃ rāsabhena sambharati

Then (over) the ass, with, 'The male carrying Agni, the male,'--for Agni is a male, and the he-ass is a male: that male carries the male;--'the sea-born child of the waters,'--for he (Agni) is the sea-born child of the waters. He thereby equips him with the ass.