Bhagwad Gita
Chapter 17, Verse 16
मनःप्रसादः सौम्यत्वं मौनमात्मविनिग्रहः।भावसंशुद्धिरित्येतत्तपो मानसमुच्यते।।17.16।।
And serenity, simplicity, gravity, self-control and purity of thought are the austerities of the mind.