Bhagavad Gita Chapter 17, Verse 24
तस्मादोमित्युदाहृत्य यज्ञदानतपःक्रियाः | प्रवर्तन्ते विधानोक्ताः सततं ब्रह्मवादिनाम् ||
tasmād oṁ ity udāhṛtya yajña-dāna-tapaḥ-kriyāḥ pravartante vidhāna-uktāḥ satataṁ brahma-vādinām
Therefore, uttering 'OM,' the actions of sacrifice, charity, and austerity are always begun by knowers of Brahman, as prescribed by scriptural injunctions.
Krishna explains how OM is used in spiritual practices. 'Tasmād oṁ ity udāhṛtya'—therefore (tasmāt), uttering (udāhṛtya) 'OM' (oṁ iti). 'Yajña-dāna-tapaḥ-kriyāḥ pravartante'—the actions (kriyāḥ) of sacrifice (yajña), charity (dāna), and austerity (tapas) begin (pravartante). 'Vidhāna-uktāḥ satataṁ brahma-vādinām'—as prescribed by scriptural injunctions (vidhāna-uktāḥ), always (satatam) by knowers of Brahman (brahma-vādinām). OM is the sacred syllable that begins all spiritual practices. When you utter OM before sacrifice, charity, or austerity, you connect them to the Supreme, sanctifying them. Knowers of Brahman always begin their practices with OM. Understanding this helps you recognize the importance of OM in sanctifying your practices.