Manava Dharmashastra 2.74

2.74

brahmanaḥ praṇavaṃ kuryād ādāv ante ca sarvadā
sravaty anoṃkṛtaṃ pūrvaṃ parastāc ca viśīryati //

Let him always pronounce the syllable Om at the beginning and at the end of (a lesson in) the Veda; (for) unless the syllable Om precede (the lesson) will slip away (from him), and unless it follow it will fade away.