Bhagavad Gita 17.16

मन: प्रसाद: सौम्यत्वं मौनमात्मविनिग्रह: |
भावसंशुद्धिरितयेत्तपो मानसमुच्यते

Translation

Serenity of thought, gentleness, silence, self-control, and purity of purpose—all these are declared as austerity of the mind.