Chapter 17 · The Three Kinds of Faith · Krishna said
Bhagavad Gita 17.24 — Beginning with Om
तस्मादोमित्युदाहृत्य यज्ञदानतपःक्रियाः। प्रवर्तन्ते विधानोक्ताः सततं ब्रह्मवादिनाम् ॥
tasmādomityudāhṛtya yajñadānatapaḥkriyāḥ | pravartante vidhānoktāḥ satataṃ brahmavādinām ||
Meaning
That is why followers of the Vedas always begin their prescribed acts of sacrifice, charity and austerity by uttering 'Om'.