Brihadaranyaka Upanishad 5.14.1

5.14.1

bhūmirantarikṣaṃ dyaurityaṣṭāvakṣarāṇyaṣṭākṣaram̐ ha vā ekaṃ gāyatryai padametadu haivāsyā etatsa yāvadeṣu triṣu lokeṣu tāvaddha jayati yo'syā etadevaṃ padaṃ veda

'Bhumi' (the earth), 'Antariksha' (sky) and 'Dyaus' (heaven) make eight syllables, and the first foot of the Gâyatrifootnote{Gâyatri (or Sâvitri) is the most sacred verse of the Vedas. It reads as follows: Tat saviturvareṇyam, bhargo devasya dhîmahi, dhiyo yo nah prachodayât—'We meditate on the adorable glory of the radiant sun. May he direct our intellect! (Ṛi. III. lxii. 10). There is also a metre called Gâyatri which has three feet, of eight syllables each. It will be seen that the verse Gâyatri is in this metre. Shankara seems to have both these senses in mind.} has eight syllables. So the above three worlds constitute the first foot of the Gâyatri. He who knows the first foot of the Gâyatri to be such wins as much as there is in those three worlds.