Bhagavad Gita · Chapter 17: The Three Kinds of Faith

Bhagavad Gita 17.5

aśāstravihitaṃ ghoraṃ tapyante ye tapo janāḥ | dambhāhaṅkārasaṃyuktāḥ kāmarāgabalānvitāḥ ||

Those who practise fierce austerities that scripture does not prescribe, full of hypocrisy and ego, and driven by the force of desire and passion,

Modern context

Some people practise fierce austerities not prescribed by any tradition, out of pride and the need to prove themselves. Extreme fasting, self-punishment, harsh discipline done to impress or to feel superior: the Gita does not admire these. It is a sober warning against spiritual extremism. Austerity that harms the body and feeds the ego misses the point entirely.