Shatapatha Brahmana 4.5.1.15

4.5.1.15

tasya hiraṇyaṃ dakṣiṇā | āgneyo vā eṣa yajño bhavatyagne reto hiraṇyaṃ tasmāddhiraṇyaṃ dakṣiṇānaḍvānvā sa hi vahenāgneyo 'gnidagdhamiva hyasya vaham bhavati

The priests’ fee for it is gold; for this is a sacrifice to Agni, and gold is Agni's seed: therefore the priests’ fee is gold. Or an ox, for such a one is of Agni's nature as regards its shoulder, since its shoulder (bearing the yoke) is as if burnt by fire.