Bhagavad Gita Chapter 17, Verse 14

देवद्विजगुरुप्राज्ञपूजनं शौचमार्जवम् | ब्रह्मचर्यमहिंसा च शारीरं तप उच्यते ||

deva-dvija-guru-prājña-pūjanaṁ śaucam ārjavam brahmacaryam ahiṁsā ca śārīraṁ tapa ucyate

Worship of deities, Brahmins, teachers, and wise people, purity, straightforwardness, celibacy, and non-violence—this is called physical austerity.

Krishna begins describing the three kinds of austerity (tapas), starting with physical austerity. 'Deva-dvija-guru-prājña-pūjanaṁ'—worship (pūjanam) of deities (deva), Brahmins (dvija), teachers (guru), and wise people (prājña). 'Śaucam ārjavam'—purity (śaucam) and straightforwardness (ārjavam). 'Brahmacaryam ahiṁsā ca'—celibacy (brahmacaryam) and non-violence (ahiṁsā). 'Śārīraṁ tapa ucyate'—this is called (ucyate) physical austerity (śārīraṁ tapas). Physical austerity involves respecting higher principles, maintaining purity, being honest, controlling senses, and practicing non-violence. This is the foundation of austerity—discipline of the body and senses. Understanding this helps you recognize physical austerity and practice it.