Vaishnava Dharmashastra 55.18

55.18

kṣaranti sarvā vaidikyo juhotiyajatikriyāḥ
akṣaraṃ tv akṣaraṃ jñeyaṃ brahmā caiva prajāpatiḥ //

All religious acts ordained in the Veda, (whether) consisting in burnt-oblations or sacrifices (or alms-giving or other pious observances), perish (after the merit obtained by them has been exhausted); but the syllable Om (akshara) must be known to be imperishable (akshara), as it is identical with Brahman, the lord of creatures.