Bg 17.24

tasmad om ity udahrtya
yajna-dana-tapah-kriyah
pravartante vidhanoktah
satatam brahma-vadinam

Word for word: 
tasmat — therefore; om — beginning with om; iti — thus; udahrtya — indicating; yajna — of sacrifice; dana — charity; tapah — and penance; kriyah — performances; pravartante — begin; vidhana-uktah — according to scriptural regulation; satatam — always; brahma-vadinam — of the transcendentalists.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Therefore, transcendentalists undertaking performances of sacrifice, charity and penance in accordance with scriptural regulations begin always with om, to attain the Supreme.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Om tad visnoh paramam padam (Rg Veda 1.22.20). The lotus feet of Visnu are the supreme devotional platform. The performance of everything on behalf of the Supreme Personality of Godhead assures the perfection of all activity.