Māndukya 4.05

सर्वस्य प्रणवो ह्यादिर्मध्यमन्तस्तथैव च। एवं हि प्रणवं ज्ञात्वा व्यश्नुते तदनन्तरम् ॥५॥

sarvasya praṇavo hy ādir madhyam antas tathaiva ca। evaṃ hi praṇavaṃ jñātvā vyaśnute tadanantaram ॥5॥

[सर्वस्य (sarvasya) - of all; प्रणवः (praṇavaḥ) - the syllable Om; हि (hi) - indeed; आदि: (ādiḥ) - beginning; मध्यम् (madhyam) - middle; अन्तः (antaḥ) - end; तथैव (tathaiva) - just so; च (ca) - and; एवं (evaṃ) - thus; हि (hi) - indeed; प्रणवम् (praṇavam) - the syllable Om; ज्ञात्वा (jñātvā) - having known; व्यश्नुते (vyaśnute) - attains; तत् (tat) - that; अनन्तरम् (anantaram) - immediately after;]

The Praṇava is truly the beginning, middle, and end of all things. Thus, knowing Praṇava in this way, one immediately attains that (ultimate reality) thereafter.