Chandogya Upanishad 4.10

upakosalo ha vai kāmalāyanaḥ satyakāme jābāle brahmacaryam uvāsa tasya ha dvādaśa vārṣāny agnīn paricacāra sa ha smānyān antevāsinaḥ samāvartayaṃs taṃ ha smaiva na samāvartayati

Upakosala Kâmalâyana dwelt as a Brahmakârin (religious student) in the house of Satyakâma Gâbâla. 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.

taṃ jāyovāca tapto brahmacārī kuśalam agnīn paricacārīt mā tvā agnayaḥ paripravocan prabrūhy asmā iti tasmai hāprocyaiva pravāsāṃcakre

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.

sa ha vyādhinā anaśituṃ dadhre tam ācāryajāyā uvāca brahmacārinn aśāna kiṃnu na aśnāsi iti sa ha uvāca bahava ime 'smin puruṣe kāmā nānātyāyāḥ vyādhībhīḥ pratipūrṇo 'smi na aśiṣyāmi iti

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.'

atha hāgnayaḥ samūdire tapto brahmacārī kuśalaṃ naḥ paryacārīt hantāsmai prabravāmeti tasmai hocuḥ prāṇo brahma kaṃ brahma khaṃ brahmeti

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:

sa hovāca vijānāmy ahaṃ yat prāṇo brahma kaṃ ca tu khaṃ ca na vijānāmīti te hocuḥ yad vāva kaṃ tad eva kham yad eva khaṃ tad eva kam iti prāṇaṃ ca hāsmai tad ākāśaṃ cocuḥ

'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).