ChU 4.13.1
अथ हैनम् आहवनीयो ऽनुशशास प्राण आकाशो द्यौर् विद्युद् इति । य एष विद्युति पुरुषो दृश्यते सो ऽहम् अस्मि स एवाहम् अस्मीति
atha hainam āhavanīyo 'nuśaśāsa prāṇa ākāśo dyaur vidyud iti | ya eṣa vidyuti puruṣo dṛśyate so 'ham asmi sa evāham asmīti
Devanagari rendered by HREEM from the edition's roman (IAST) text; the IAST is the edition's own.
Then the Âhavanîya fire taught him: 'Breath, ether, heaven, and lightning (these are my forms). The person that is seen in the lightning, I am he, I am he indeed.
Translated by F. Max Müller