Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 38
Sanskrit:
अत्रापि बह्वृचैर्गीतं शृणु मेऽसुरसत्तम ।सत्यमोमिति यत् प्रोक्तं यन्नेत्याहानृतं हि तत् ॥ ३८ ॥
ITRANS:
atrāpi bahvṛcair gītaṁśṛṇu me ’sura-sattamasatyam om iti yat proktaṁyan nety āhānṛtaṁ hi tat
Translation:
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 Bahvṛca-śruti, which says that a promise is truthful preceded by the word om and untruthful if not.