Bhagavad Gita · Chapter 17: The Three Kinds of Faith

Bhagavad Gita 17.24

tasmādomityudāhṛtya yajñadānatapaḥkriyāḥ | pravartante vidhānoktāḥ satataṃ brahmavādinām ||

Therefore those who follow the Veda always begin the acts of sacrifice, giving, and austerity that scripture prescribes by saying "Om."

Modern context

Every act of sacrifice, giving and discipline begins with Om. It is a way of turning one's attention to the highest before beginning. A small sound at the start of an action can change its spirit. Many traditions have something similar: a prayer before a meal, a moment of silence before a speech, a word of dedication before work.