Maitri (Maitrāyaṇīya) Upaniṣad 2.1

2.1

atha bhagavāñchākāyanyaḥ suprīto'bravīdrājānaṃ mahārāja bṛhadrathekṣvākuvaṃśadhvajaśīrṣātmajaḥ kṛtakṛtyastvaṃ marunnāmno viśruto'sītyayaṃ vā va khalvātmā te katamo bhagavānvarṇya iti taṃ hovāca iti ||

1. Then the honorable Śākāyanya, well pleased, said to the king: ‘Great king Bṛihadratha, banner of the family of Ikshvāku, speedily will you who are renowned as “Swift Wind” (Marut) attain your purpose and become a knower of the Soul (Ātman)!

This one, assuredly, indeed, is your own self (*ātman*).’

‘Which one is it, Sir?’

Then he said to him:—