Bhagavad Gita · 17.16

मनःप्रसादः सौम्यत्वं मौनमात्मविनिग्रहः।भावसंशुद्धिरित्येतत्तपो मानसमुच्यते॥

manaḥ-prasādaḥ saumyatvaṁ maunam ātma-vinigrahaḥ bhāva-saṁśhuddhir ity etat tapo mānasam uchyate

Serenity of mind, gentleness, silence, self-control, and purity of heart — this is called austerity of the mind.

— Bhagavad Gita 17.16

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)