CHĀNDOGYA UPANISHAD · 4.10.4

अथ हाग्नयः समूदिरे ।
तप्तो ब्रह्मचारी कुशलं नः पर्यचारीत् ।
हन्तास्मै प्रब्रवामेति तस्मै होचुः ।
प्राणो ब्रह्म कं ब्रह्म खं ब्रह्मेति ॥

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: 'Breath is Brahman, Ka (pleasure) is Brahman, Kha (ether) is Brahman.'

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)