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मनः प्रसादः सौम्यत्वं मौनमात्मविनिग्रहः ।
भावसंशुद्धिरित्येतत्तपो मानसमुच्यते ॥१७- १६॥

manaH prasaadaH saumyatwaM maunamaatmawinigrahaH ।

bhaawasaMshuddhirityetattapo maanasamuchyate ॥१७- १६॥

Source: Automatic ITRANS Transliteration

Mental serenity, gentleness, silence, self-restraint,
Purity of emotion. So are these called austerity of thought.

Source: Milind S. Pandit

The serenity of mind, gentleness, silence, self-restraint, and the purity of mind are called the austerity of thought.

Source: The Bhagavad-Gita: The Sacred Song

And serenity, simplicity, gravity, self-control and purity of thought are the austerities of the mind.

Source: Bhagavad Gita As It Is