One verse

Bhagavad Gita 17.6

17.6

कर्षयन्तः शरीरस्थं भूतग्राममचेतसः | मां चैवान्तःशरीरस्थं तान्विद्ध्यासुरनिश्चयान् ||१७-६||

karṣayantaḥ śarīrasthaṃ bhūtagrāmamacetasaḥ . māṃ caivāntaḥśarīrasthaṃ tānviddhyāsuraniścayān ||17-6||

karṣayantaḥ
tormenting
śarīra-stham
situated within the body
bhūta-grāmam
the combination of material elements
acetasaḥ
having a misled mentality
mām
Me
ca
also
eva
certainly
antaḥ
within
śarīra-stham
situated in the body
tān
them
viddhi
understand
āsura-niścayān
demons

Those who undergo severe austerities and penances not recommended in the scriptures, performing them out of pride and egoism, who are impelled by lust and attachment, who are foolish and who torture the material elements of the body as well as the Supersoul dwelling within, are to be known as demons.