Shatapatha Brahmana 2.5.1.20

2.5.1.20

navaprayājam bhavati | navānuyājaṃ daśākṣarā vai virāḍathaitāmubhayato nyūnāṃ virājaṃ karoti prajananāyaitasmādvā ubhayato nyūnātprajananātprajāpatiḥ prajāḥ sasṛja itaścordhvā itaścāvācīstatho evaiṣa etasmādubhayata eva nyūnātprajananātprajāḥ sṛjata itaścordhvā itaścāvācīstasmānnavaprayājam bhavati navānuyājam

[At the Vaisvadeva-offering] there are nine fore-offerings and nine after-offerings. Now the virâg metre consists of ten syllables: hence, he obtains both times an inferior (incomplete) virâg for the sake of production, because it was from that inferior (lower) source of production that Pragâpati twice produced creatures--both the upright and those looking to the ground. This is why (the Vaisvadeva) has nine fore-offerings and nine after-offerings.