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SB 11.16.12

hiraṇyagarbho vedānāṁ
 mantrāṇāṁ praṇavas tri-vṛt
akṣarāṇām a-kāro ’smi
 padāni cchandasām aham
Synonyms: 
hiraṇya-garbhaḥ — Lord Brahmā; vedānām — of the Vedas; mantrāṇām — of mantras; praṇavaḥ — the oṁkāra; tri-vṛt — consisting of three letters; akṣarāṇām — of letters; a-kāraḥ — the first letter, a; asmi — I am; padāni — the three-line Gāyatrī mantra; chandasām — among sacred meters; aham — I am.
Translation: 
Among the Vedas I am their original teacher, Lord Brahmā, and of all mantras I am the three-lettered oṁkāra. Among letters I am the first letter, “a,” and among sacred meters I am the Gāyatrī mantra.