śraddhātrayavibhāgayoga - Verse śloka 24
śloka 24
tasmādomityudāhṛtya yajñadānatapaḥkriyāḥ |
pravartante vidhānoktāḥ satataṃ brahmavādinām ||17-24||
Translation
Therefore, the actions of sacrifice, charity and austerity as prescribed by scripture for those who know Brahman always begin with the utterance of Om.