Māndukya 4.06

प्रणवं हीश्वरं विद्यात् सर्वस्य हृदये स्थितम्। सर्वव्यापिनमोङ्कारं मत्वा धीरो न शोचति ॥६॥

praṇavaṃ hīśvaraṃ vidyāt sarvasya hṛdaye sthitam। sarvavyāpinamoṅkāraṃ matvā dhīro na śocati ॥6॥

[प्रणवम् (praṇavam) - praṇava; (the syllable Om;) हि (hi) - indeed; ईश्वरम् (īśvaram) - the Lord; विद्यात् (vidyāt) - one should know; सर्वस्य (sarvasya) - of all; हृदये (hṛdaye) - in the heart; स्थितम् (sthitam) - situated; सर्वव्यापि (sarvavyāpi) - all-pervading; नमः (namaḥ) - obeisance; ओङ्कारम् (oṅkāram) - Omkara; (the syllable Om;) मत्वा (matvā) - having known; धीरो (dhīraḥ) - the wise one; न (na) - not; शोचति (śocati) - grieves;]

One should know that the Praṇava is truly the Lord, present in the heart of all. Having known the all-pervading Omkāra, with obeisance, the wise do not grieve.