Maitri (Maitrāyaṇīya) Upaniṣad 6.21

6.21

athānyatrāpyuktam ūrdhvagā nāḍī suṣumnākhyā prāṇasañcāriṇī tālvantarvicchinnā tayā prāṇoṅkāramanoyuktayordhvamutkramet tālvadhyāgraṃ parivartya indriyāṇyasaṃyojya mahimā mahimānaṃ nirīkṣetā tato nirātvakameti nirātmakatvānna sukhaduḥkhabhāgbhavati kevalatvaṃ labhatā ityevaṃ hyāha
paraḥ pūrvaṃ pratiṣṭhāpya nigṛhītānilaṃ tataḥ |
tīrtvā pāramapāreṇa paścādyuñjīta mūrdhvani ||

21. Now, it has elsewhere been said: 'There is an artery, called the Sushumnā, leading upwards, conveying the breath, piercing through the palate. Through it, by joining ($\surd$yuj) the breath, the syllable *Om*, and the mind, one may go aloft. By causing the tip of the tongue to turn back against the palate and by binding together (*saṁ-yojya*) the senses, one may, as greatness, perceive greatness.' Thence he goes to selflessness. Because of selflessness, one becomes a non-experiencer of pleasure and pain; he obtains the absolute unity (*kevalatva*). For thus has it been said:—

After having first caused to stand still
The breath that has been restrained, then,
Having crossed beyond the limited, with the unlimited
One may at last have union in the head.