Bg 17.17

sraddhaya paraya taptam
tapas tat tri-vidham naraih
aphalakanksibhir yuktaih
sattvikam paricaksate

Word for word: 
sraddhaya — with faith; paraya — transcendental; taptam — executed; tapah — austerity; tat — that; tri-vidham — of three kinds; naraih — by men; aphala-akanksibhih — who are without desires for fruits; yuktaih — engaged; sattvikam — in the mode of goodness; paricaksate — is called.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This threefold austerity, performed with transcendental faith by men not expecting material benefits but engaged only for the sake of the Supreme, is called austerity in goodness.