Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.1.21

2.2.1.21

tasyai dhenurdakṣiṇā | dhenuriva vā iyam manuṣyebhyaḥ sarvān kāmānduhe mātā dhenurmāteva vā iyam manuṣyānbibharti tasmāddhenurdakṣiṇaitannvekamayanam

The priests' fee for (offering to) her consists of a cow; for this (earth) is, as it were, a cow: she milks out for men all their desires. The cow is a mother, and this (earth) also is a mother, for she bears the men: for this reason the priests' fee is a cow. This is one mode (of performing those offerings).