By Rig Veda (Mandala 3.62.10)
There are many different musical arrangements of this powerful mantra.
The Gayatri Mantra consists of 24 syllables - three lines of eight syllables each. The first line (Om Bhur Bhuva Svaha) is considered an invocation, and is not technically a part of the original Gayatri Mantra as it appears in the Upanishads.
Om bhur bhuva svaha
tat savitur varenyam bhargo devasya dhimahi dhiyo yo nah prachodayat
Oh God, the Protector, the basis of all life, who is self-existent, who is free from all pains and whose contact frees the soul from all troubles, who pervades the Universe and sustains all, the Creator and Energizer of the whole Universe, the giver of happiness, who is worthy of acceptance, the most excellent, who is pure and the purifier of all, let us embrace that very God, so that our mental faculties may be used in the right direction.