Srimad Bhagavatam Canto 08, Chapter 19, Text 38

SB 8.19.38

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.
Srimad Bhagavatam Canto 08, Chapter 19, Text 37
Srimad Bhagavatam Canto 08, Chapter 19, Text 39