Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.15

3.4.1.15

navakapālaḥ puroḍāśo bhavati | śiro vai yajñasyātithyaṃ navākṣarā vai gāyatryaṣṭau tāni yānyanvāha praṇavo navamaḥ pūrvārdho vai yajñasya gāyatrī pūrvārdhe eṣa yajñasya tasmānnavakapālaḥ puroḍāśo bhavati

It is a sacrificial cake on nine potsherds;--for the guest-offering is the head of the sacrifice, and the Gâyatrî consists of nine syllables: eight (syllables) are those he recites and the sacred syllable is the ninth; and the Gâyatrî is the fore-part of the sacrifice, and so is that (cake) the fore-part of the sacrifice: therefore it is a cake on nine potsherds.