atrapi bahvrcair gitam
srnu me ’sura-sattama
satyam om iti yat proktam
yan nety ahanrtam hi tat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
One might argue that since you have already promised, how can you refuse? O best of the demons, just take from me the evidence of the Bahvrca-sruti, which says that a promise is truthful preceded by the word om and untruthful if not.