Chapter 17 · The Three Kinds of Faith · Krishna said
Bhagavad Gita 17.5 — Austerity against scripture
अशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाः। दम्भाहंकारसंयुक्ताः कामरागबलान्विताः ॥
aśāstravihitaṃ ghoraṃ tapyante ye tapo janāḥ | dambhāhaṃkārasaṃyuktāḥ kāmarāgabalānvitāḥ ||
Meaning
Those who practise harsh austerities not taught in scripture, full of show and ego, driven by desire, attachment and force —