Khandogya-Upanishad: Sixth Prapathaka, Seventh Khanda
1. “Man (purusha), my son, consists of sixteen parts. Abstain from food for fifteen days, but drink as much water as you like, for breath comes from water, and will not be cut off, if you drink water.”
2. Svetaketu abstained from food for fifteen days. Then he came to his father and said: “What shall I say?” The father said: “Repeat the Rik, Yagus, and Saman verses.” He replied: “They do not occur to me, Sir.”
3. The father said to him: “As of a great lighted fire one coal only of the size of a firefly may be left, which would not burn much more than this (i.e. very little), thus, my dear son, one part only of the sixteen parts (of you) is left, and therefore with that one part you do not remember the Vedas. Go and eat!
4. “Then wilt thou understand me.” Then Svetaketu ate, and afterwards approached his father. And whatever his father asked him, he knew it all by heart. Then his father said to him:
5. “As of a great lighted fire one coal of the size of a firefly, if left, may be made to blaze up again by putting grass upon it, and will thus burn more than this,
6. “Thus, my dear son, there was one part of the sixteen parts left to you, and that, lighted up with food, burnt up, and by it you remember now the Vedas.” After that, he understood what his father meant when he said: “Mind, my son, comes from food, breath from water, speech from fire.” He understood what he said, yea, he understood it.