Bhagavad Gita · 17.5

अशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाः।दम्भाहङ्कारसंयुक्ताः कामरागबलान्विताः॥

aśhāstra-vihitaṁ ghoraṁ tapyante ye tapo janāḥ dambhāhaṅkāra-saṁyuktāḥ kāma-rāga-balānvitāḥ

Those who practice terrible austerities not enjoined by the scriptures, driven by hypocrisy and ego, impelled by the force of desire and attachment —

— Bhagavad Gita 17.5

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)