Bhagwad Gita
Chapter 17, Verse 5
अशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाः।दम्भाहङ्कारसंयुक्ताः कामरागबलान्विताः।।17.5।।
Those men who practice terrific austerities not prescribed by the scriptures, given to hypocrisy and egoism, driven by the force of lust and attachment.