śhraddhayā parayā taptaṁ tapas tat tri-vidhaṁ naraiḥ
aphalākāṅkṣhibhir yuktaiḥ sāttvikaṁ parichakṣhate||17||

Meaning : Austerity (tapas) done with supreme faith, by people who are free from desire for results, and done with discipline that kind of austerity is called sattvic (pure).