Khandogya-Upanishad: Fourth Prapathaka, Tenth Khanda
1. Upakosala Kamaliyana dwelt as a Brahmakarin (religious student) in the house of Satyakama Gabala. He tended his fires for twelve years. But the teacher, though he allowed other pupils (after they had learnt the sacred books) to depart to their own homes, did not allow Upakosala to depart.
2. Then his wife said to him: “This student, who is quite exhausted (with austerities), has carefully tended your fires. Let not the fires themselves blame you, but teach him.” The teacher, however, went away on a journey without having taught him.
3. The student from sorrow was not able to eat. Then the wife of the teacher said to him: “Student, eat! Why do you not eat?” He said: “There are many desires in this man here, which lose themselves in different directions. I am full of sorrows, and shall take no food.”
4. Thereupon the fires said among themselves “This student, who is quite exhausted, has carefully tended us. Well, let us teach him.” They said to him:
5. “Breath is Brahman, Ka (pleasure) is Brahman, Kha (ether) is Brahman.”
He said: “I understand that breath is Brahman, but I do not understand Ka or Kha.”
They said: “What is Ka is Kha, what is Kha is Ka.” They therefore taught him Brahman as breath, and as the ether (in the heart).