Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 12
Sanskrit:
हिरण्यगर्भो वेदानां मन्त्राणां प्रणवस्त्रिवृत् ।अक्षराणामकारोऽस्मि पदानिच्छन्दसामहम् ॥ १२ ॥
ITRANS:
hiraṇyagarbho vedānāṁmantrāṇāṁ praṇavas tri-vṛtakṣarāṇām a-kāro ’smipadāni cchandasām aham
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.