The works of Ādi Śaṅkarācārya

Viṣṇusahasranāmastotram

Verse 177

सर्वस्य प्रणवो ह्यादिः मध्यमन्तस्तथैव च ।एवं हि प्रणवं ज्ञात्वा व्यश्नुते तदनन्तरम् ॥

In plain words

Oṃ is where everything begins, Oṃ is what everything stands in, Oṃ is where everything ends. Know Oṃ in just this way, and you reach it — with no interval at all.

Close to the Sanskrit

Of all, the praṇava is indeed the beginning, the middle, and the end likewise. Thus indeed, having known the praṇava, one attains it immediately thereafter.