Bhagavad Gita 17.14
Worship of God and the Master; respect for the preacher and the philosopher; purity, rectitude, continence, and harmlessness—all of these constitute physical austerity.
देवद्विजगुरुप्राज्ञपूजनं शौचमार्जवम्।ब्रह्मचर्यमहिंसा च शारीरं तप उच्यते॥devadvijaguruprājñapūjanaṃ śaucamārjavam|brahmacaryamahiṃsā ca śārīraṃ tapa ucyate||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).