Kaṭha Upaniṣad 3.1

3.1

ṛtaṃ pibantau sukṛtasya loke guhāṃ praviṣṭau parame parārdhe /
chāyātapau brahmavido vadanti pañcāgnayo ye ca triṇāciketāḥ //

1. There are two that drink of righteousness (_ṛta_) in the world of good deeds ;
Both are entered into the secret place [of the heart], and in the highest upper sphere.
Brahma-knowers speak of them as ‘light’ and ‘shade,’
And so do householders who maintain the five sacrificial fires, and those too who perform the triple Naciketas-fire.