Katha Upanishad - Verse 1.3.1
Text: 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 || 1. The two, who enjoy the fruits of their good deeds, being lodged in the cavity of the seat of the supreme, the knowers of Brahman call shadow and light, as also those who maintain five fires and have thrice propitiated the Nachiketa fire....