Bhagavad Gita · Akshara Brahma YogaChapter 8, Shloka 11 (8.11)

Shankaracharya — the worship of Om, for those of dull and middling understanding. Yat akṣaraṃ … avināśi veda-vidaḥ … vadanti — evidence Brihadaranyaka 3.8.8 (etad akṣaraṃ gārgi brāhmaṇā abhivadanti … asthūlam anaṇu); yatayaḥ … saṃnyāsinaḥ vītarāgāḥ; brahmacaryaṃ gurau caranti — brahmacharya lived with the guru. Then a long context — Prashna Up. 5 and Kathopanishad 1.2.14–15 — and the conclusion: parasya brahmaṇo vācaka-rūpeṇa pratimāvat pratīka-rūpeṇa vā … manda-madhyama-buddhīnāṃ vivakṣitasya oṃkārasya upāsanaṃ kālāntare mukti-phalam — Om, as the name of the supreme Brahman or as a symbol like an image, is a form of worship for those of dull and middling understanding, whose fruit is liberation in due course of time.

Acharya Ramanuja. Yad akṣaram asthūlatvādi-guṇakaṃ veda-vido vadanti — that which they call the imperishable, with qualities such as not being gross; padyate gamyate anena iti padaṃ, tad nikhila-vedānta-vedyaṃ mat-svarūpam akṣaraṃ yathā upāsyaṃ tathā saṃkṣepeṇa pravakṣyāmi — "pada" means that by which one reaches; I shall tell you briefly how my imperishable nature, to be known through all of Vedanta, is to be worshipped.