Bg 17.24

tasmād oṁ ity udāhṛtya
yajña-dāna-tapaḥ-kriyāḥ
pravartante vidhānoktāḥ
satataṁ brahma-vādinām
Synonyms: 
tasmāt — therefore; oṁ — beginning with oṁ; iti — thus; udāhṛtya — indicating; yajña — of sacrifice; dāna — charity; tapaḥ — and penance; kriyāḥ — performances; pravartante — begin; vidhāna-uktāḥ — according to scriptural regulation; satatam — always; brahma-vādinām — of the transcendentalists.
Translation: 
Therefore, transcendentalists undertaking performances of sacrifice, charity and penance in accordance with scriptural regulations begin always with oṁ, to attain the Supreme.
Purport: 

Oṁ tad viṣṇoḥ paramaṁ padam (Ṛg Veda 1.22.20). The lotus feet of Viṣṇu are the supreme devotional platform. The performance of everything on behalf of the Supreme Personality of Godhead assures the perfection of all activity.