The Sacred Gayatri Mantra - Brihadaranyaka Upanishad

The Sacred Gayatri Mantra—Chapter XIV (Brihadaranyaka Upanishad)
1. The words Bhumi (earth), Antariksha (sky) and Dyaus (heaven) form eight syllables and the first foot of the Gayatri consists of eight syllables. So the three worlds constitute the first foot of the Gayatri. Whosoever knows this about the first foot of the Gayatri wins all that is in the three worlds.

2. Richah, Yajumshi and Samani form eight syllables and the second foot of the Gayatri consists of eight syllables. So these three Vedas constitute the second foot of the Gayatri. Whosoever thus knows the second foot of the Gayatri wins as much as that treasury of knowledge, the three Vedas, has to confer.

3. Prana, apana and vyana form eight syllables and the third foot of the Gayatri. consists of eight syllables. So these three forms of the vital breath constitute the third foot of the Gayatri. Whosoever knows this about the third foot of the Gayatri wins all the living beings that are in the universe. Now, its turiya, apparently visible (darsata) and supramundane (paroraja) foot is this—sun that glows yonder. That which is fourth is called turiya. He (the being in the solar orb) is apparently visible (darsata), because he is seen, as it were, by the yogis. He is supramundane (paroraja), because he shines alone on the whole universe as its overlord. He who thus knows the fourth foot of the Gayatri shines with splendour and glory.

4. That Gayatri rests on that fourth, apparently visible, supramundane foot. And that, again,rests on truth. The eye is truth, for the eye is indeed truth. Therefore, even today, if two persons come disputing, one saying: "I saw it," and another: "I heard of it," we should trust the one who says: "I saw it. That truth rests on strength. The vital breath (prana) is strength. Hence truth rests on the vital breath. Therefore they say that strength is more powerful than truth. Thus the Gayatri is based on the vital breath within the body. That Gayatri protected the gayas. The organs are the gayas; therefore the Gayatri protected (tatre) the organs. Because it protected the organs, it is called the Gayatri. The Savitri verse, which the teacher communicates to the pupil, is no other than this. It saves the organs of the pupil to whom it is imparted by the teacher.

5. Some impart to the pupil the Savitri which is in the Anushtubh metre, saying: "The goddess of speech is Anushtubh; so we shall impart it to him." But one should not do that. One should impart only that Savitri which is Gayatri. Verily, if one who knows this accepts too much as a gift, as it were, it is not enough for even one foot of the Gayatri.

6. If he (the knower of the Gayatri) accepts as a gift the three worlds full of wealth, he will be receiving the fruit of knowing only the first foot of the Gayatri. If he accepts as a gift as much as this treasury of knowledge, the Vedas, has to confer, he will be receiving the fruit of knowing only the second foot of the Gayatri. And if he accepts as a gift as much as is covered by all living creatures in the world, he will be receiving the fruit of knowing only the third foot of the Gayatri. While the fruit of knowing its fourth, apparently visible, supramundane foot—yonder sun that glows—is not to be counterbalanced by any gift received. Indeed, how could anyone receive so much as a gift?

7. The salutation to the Gayatri:
  "O Gayatri, thou art one—footed, two—footed, three—footed and four—footed. And thou art without any feet, for thou art unattainable. Salutation to thee, fourth foot, apparently visible and supramundane! May the enemy never attain his object!" Should the knower of the Gayatri bear hatred towards anyone, he should either use this mantra: "May his desired object never flourish!"—in which case that object of the person against whom he thus salutes the Gayatri never flourishes—or he may say: "May I attain that cherished object of his!"

8. On this subject Janaka, Emperor of Videha, said to Budila, the son of Asvatarasva: "Well, how is it that you, who called yourself a knower of the Gayatri, have come to he an elephant and are carrying me?" 
He replied: "Because, Your Majesty, I did not know its mouth." Janaka said: "Fire is its mouth. If people put a large quantity of fuel into the fire, it is all burnt up. Similarly, a man who knows this, even if he commits a great many sins, consumes them all and becomes pure, clean and free from decay and death."