Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.3.28

2.2.3.28

athaitadeva paro 'kṣaṃ rūpam | yadeva purastādvāti tadvasantasya rūpaṃ yatstanayati tadgrīṣmasya yadvarṣati tadvarṣāṇāṃ yadvidyotate tacarado yadvṛṣṭvodgṛhṇāti taddhemantasya varṣāḥ sarva ṛtava ṛtūnprāviśadṛtubhya evainametannirmimīte

The priests' fee for this (sacrifice) consists of gold, This sacrifice belongs to Agni, and gold is Agni's seed: this is why the priests' fee consists of gold. Or it may be an ox; for the latter is of the nature of Agni as far as its shoulder is concerned, since its shoulder (by carrying the yoke) is as if burnt by fire. Moreover, Agni is oblation-bearer to the gods, and that (ox) bears (or draws, loads) for men: this is why an ox may be given as the priests' fee.