Bhagavad Gita 17.24

तस्माद् ॐ इतुदाहृत्य यज्ञदानतप: क्रिया: |
प्रवर्तन्ते विधानोक्ता: सततं ब्रह्मवादिनाम्

Translation

Therefore, when performing acts of sacrifice, offering charity, or undertaking penance, expounders of the Vedas always begin by uttering “Om” according to the prescriptions of Vedic injunctions.