Bhagavad Gita Chapter 17.24
tasmād oḿ ity udāhṛtya
yajña-dāna-tapaḥ-kriyāḥ
pravartante vidhānoktāḥ
satataḿ brahma-vādinām
Translation Therefore, transcendentalists undertaking performances of sacrifice, charity and penance in accordance with scriptural regulations begin always with om, to attain the Supreme.