SB 11.16.12
hiranyagarbho vedanam
mantranam pranavas tri-vrt
aksaranam a-karo ’smi
padani cchandasam aham
Translation:
Among the Vedas I am their original teacher, Lord Brahma, and of all mantras I am the three-lettered omkara. Among letters I am the first letter, “a,” and among sacred meters I am the Gayatri mantra.