Bhagavad Gita Chapter 17.16
manaḥ-prasādaḥ saumyatvaḿ
maunam ātma-vinigrahaḥ
bhāva-saḿśuddhir ity etat
tapo mānasam ucyate
Translation
And satisfaction, simplicity, gravity, self-control and purification of one’s existence are the austerities of the mind.