One verse

Srimad Bhagavatam 8.19.38

8.19.38

अत्रापि बह्वृचैर्गीतं श‍ृणु मेऽसुरसत्तम । सत्यमोमिति यत् प्रोक्तं यन्नेत्याहानृतं हि तत् ॥ ३८ ॥

atrapi bahvrcair gitam srnu me ’sura-sattama satyam om iti yat proktam yan nety ahanrtam hi tat

atra api
in this regard also (in deciding what is truth and what is not truth)
bahu-rcaih
by the sruti-mantras known as Bahvrca-sruti, which are evidence from the Vedas
gitam
what has been spoken
srnu
just hear
me
from me
asura-sattama
O best of the asuras
satyam
the truth is
om iti
preceded by the word om
yat
that which
proktam
has been spoken
yat
that which is
na
not preceded by om
iti
thus
aha
it is said
anrtam
untruth
hi
indeed
tat
that

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.