Bhagavad Gita 17.24
Therefore, all acts of sacrifice, gifts, and austerities prescribed by the scriptures are always begun by those who understand the Spirit with the word "Om."
तस्मादोमित्युदाहृत्य यज्ञदानतपःक्रियाः।प्रवर्तन्ते विधानोक्ताः सततं ब्रह्मवादिनाम्॥tasmādomityudāhṛtya yajñadānatapaḥkriyāḥ|pravartante vidhānoktāḥ satataṃ brahmavādinām||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).