Bhagavad-gītā 17.5-6
BG 17.5-6
Open the full passageअशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाः । दम्भाहंकारसंयुक्ताः कामरागबलान्विताः ॥५॥ कर्षयन्तः शरीरस्थं भूतग्राममचेतसः । मां चैवान्तःशरीरस्थं तान्विद्ध्यासुरनिश्चयान् ॥६॥
Sanskrit
aśāstra-vihitaṁ ghoraṁ tapyante ye tapo janāḥ dambhāhaṅkāra-saṁyuktāḥ kāma-rāga-balānvitāḥ karṣayantaḥ śarīra-sthaṁ bhūta-grāmam acetasaḥ māṁ caivāntaḥ śarīra-sthaṁ tān viddhy āsura-niścayān
Translation
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.