Bhagavad Gita · 17.24

तस्मादोमित्युदाहृत्य यज्ञदानतपःक्रियाः।प्रवर्तन्ते विधानोक्ताः सततं ब्रह्मवादिनाम्॥

tasmād om ity udāhṛitya yajña-dāna-tapaḥ-kriyāḥ pravartante vidhānoktāḥ satataṁ brahma-vādinām

Therefore, acts of sacrifice, charity, and austerity, as prescribed by the scriptures, are always begun by the knowers of Brahman with the utterance of Om.

— Bhagavad Gita 17.24

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)