Brihadaranyaka Upanishad 5.14.2

5.14.2

ṛco yajūm̐ṣi sāmānītyaṣṭāvakṣarāṇyaṣṭākṣaram̐ ha vā ekaṃ gāyatryaiai padametadu haivāsyā etatsa yāvatīyaṃ trayī vidyā tāvaddha jayati yo'syā etadevaṃ padaṃ veda

'Richah,' 'Yajūṃṣi' and 'Sāmāni'footnote{The plural forms of the names of the three Vedas.} make eight syllables, and the second foot of the Gāyatrī has eight syllables. So the above three Vedas constitute the second foot of the Gāyatrī. He who knows the second foot of the Gāyatrī to be such wins as much as that treasury of knowledge, the three Vedas, has to confer.