Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.5.21

4.2.5.21

gāyatryevaikākinī prātaḥsavanaṃ vahati | saitābhyām paṅktibhyāṃ stotrapaṅktyā ca haviṣpaṅktyā ca catvāryājyāni bahiṣpavamānam pañcamam pañcaparā paṅktiḥ saitayā stotrapaṅktyānekākinī gāyatrī prātaḥsavanam vahati

The Gâyatrî alone bears singly the morning libation,--with those two sets of five (paṅkti), the set of five chants, and the set of five oblations: there are four Âgya (chants) and the Bahishpavamâna is the fifth,--the Paṅkti metre is five-footed with that paṅkti of chants, not alone, the Gâyatrî bears the morning libation.