KAṬHA UPANISHAD · 1.3.1

ऋतं पिबन्तौ सुकृतस्य लोके
गुहां प्रविष्टौ परमे परार्धे ।
छायातपौ ब्रह्मविदो वदन्ति
पञ्चाग्नयो ये च त्रिणाचिकेताः ॥ १॥

ṛtaṃ pibantau sukṛtasya loke
guhāṃ praviṣṭau parame parārdhe ।
chāyātapau brahmavido vadanti
pañcāgnayo ye ca triṇāciketāḥ ॥ 1॥

There are two that drink deep of the Truth in the world of work well accomplished: they are lodged in the secret plane of being and in the highest kingdom of the most High is their dwelling: as of light and shade the knowers of the Brahman speak of them and those of the five fires and those who have the three fires of Nachiketas.

Sri Aurobindo · CWSA 18, p. 113