Bhagavad Gita 17.16
मनःप्रसादः सौम्यत्वं मौनमात्मविनिग्रहः। भावसंशुद्धिरित्येतत्तपो मानसमुच्यते।।17.16।।
manaḥprasādaḥ saumyatvaṃ maunamātmavinigrahaḥ| bhāvasaṃśuddhirityetattapo mānasamucyate||17.16||
manahprasadah saumyatvam maunamatmavinigrahah bhavasamshuddhirityetattapo manasamucyate17.16
Meaning
Serenity of mind, gentleness, silence, self-control, and purity of the inner nature — this is called the austerity of the mind.
मन की प्रसन्नता, सौम्यता, मौन, आत्मसंयम, और अन्तःकरण की शुद्धि; यह सब मानसिक तप कहलाता है।
This is verse 16 of 28 in Chapter 17 (Shraddhatraya Vibhaga Yoga) of the Bhagavad Gita.