Śatapatha Brāhmaṇa ("of a hundred paths") 11.2.7.30

11.2.7.30

te devāḥ etānpatnīsaṃyājānpaścātparyauhanta mithunamevaitadupariṣṭādadadhata prajātyai tadyatpatnīsaṃyājā ijyante mithunamevaitadupariṣṭāddhatte prajātyai devānāṃ ha vai prajātimanu prajāyate mithunena-mithunena ha prajāyate ya evametadveda

The gods fortified the Patnîsamyâgas by a mound from behind, and placed a couple thereon for the sake of procreation: thus when the Patnîsamyâgas are performed, he places a couple thereon for the sake of procreation; for, indeed, after the procreation of the gods offspring is produced, and offspring is produced by pair after pair (of men and beasts) for him who knows this.