Maitri (Maitrāyaṇīya) Upaniṣad 6.19

6.19

athānyatrāpyuktam yadā vai bahirvidvānmano niyamyendriyārthān ca prāṇo niveśayitvā niḥsaṅkalpastatastiṣṭhet aprāṇādiha yasmātsambhūtaḥ prāṇasaṃjñako jīvastasmātprāṇo vai turyākhye dhārayetprāṇam ityevaṃ hyāha
acittaṃ cittamadhyastamacintyaṃ guhyamuttamam |
tatra cittaṃ nidhāyeta tacca liṅgaṃ nirāśrayam ||

19. Now, it has elsewhere been said: ‘Verily, when a knower has restrained his mind from the external, and the breathing spirit (prāṇa) has put to rest objects of sense, thereupon let him continue void of conceptions. Since the living individual (jīva) who is named “breathing spirit” has arisen here from what is not breathing spirit, therefore, verily, let the breathing spirit restrain his breathing spirit in what is called the fourth condition (turya).’ For thus has it been said:—

That which is non-thought, [yet] which stands in the midst of thought,
The unthinkable, supreme mystery!—
Thereon let one concentrate his thought
And the subtile body (liṅga), too, without support.