Bhagavad Gita Chapter 17 Verse 24

Gita 17.24

तस्मात्, ओम्, इति, उदाहृत्य, यज्ञदानतपःक्रियाः,
प्रवर्तन्ते, विधानोक्ताः, सततम्, ब्रह्मवादिनाम्।।24।।

Gita 17.24

Tasmaat’, Om’, iti, udaahrtya, yagyadaantapHkriyaH,
Prvartante, vidhaanoktaH, sattam’, brahmvaadinaam’ ||24||

Translation: (Tasmaat’) Therefore (brahmvaadinaam’) of those who pray to God and (vidhaanoktH) of those who advise rituals which are in accordance with the scriptural injunctions (yagyadaantapHkriyaH) the acts of yagya, charity and austerity or sumiran[3] (sattam’) always (Om’) Om’ (iti) this mantra (udaahrtya) only by chanting (prvartante) commence. (24)

Translation: Therefore, the acts of yagya, charity and austerity and sumiran of those, who pray to God and those who advise rituals which are in accordance with the scriptural injunctions, always commence by chanting this mantra Om’ only; i.e. in the jaap of the three mantras, commencement is from Om’ mantra through breath.