Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.2.7

4.3.2.7

caturakṣarāṇi ha vā agre candāṃsyāsuḥ | tato jagatī somamacāpatatsā trīṇykṣarāṇi hitvājagāma tatastriṣṭupsomamacāpatatsaikamakṣaraṃ hitvājagāma tato gāyatrī somamacāpatatsaitāni cākṣarāṇi harantyāgacatsomaṃ ca tato 'ṣṭākṣarā gāyatryabhavattasmādāhuraṣṭākṣarā gāyatrīti

Now, in the beginning the metres consisted of four syllables. Then Gagatî flew up for Soma and came back, leaving behind three syllables. Then Trishtubh flew up for Soma and came back, leaving behind one syllable. Then Gâyatrî flew up for Soma, and she came back bringing with her those syllables as well as Soma. Thus she came to consist of eight syllables: wherefore they say, 'Gâyatrî is octosyllabic.'