Bhagavad Gita · Chapter 17: The Three Kinds of Faith

Bhagavad Gita 17.16

manaḥprasādaḥ saumyatvaṃ maunamātmavinigrahaḥ | bhāvasaṃśuddhirityetattapo mānasamucyate ||

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

Modern context

Austerity of the mind: serenity, gentleness, silence, self-control and purity of feeling. Silence here does not only mean not talking. It means inner quiet, a mind that does not chatter endlessly. Serenity of mind is listed first. A calm, cheerful mind is itself a discipline, cultivated by choosing again and again not to feed worry, resentment and agitation.