manaḥ-prasādaḥ saumyatvaṁ maunam ātma-vinigrahaḥ
bhāva-sanśhuddhir ity etat tapo mānasam uchyate||16||

Meaning : Speech that causes no disturbance, is truthful, pleasant, and beneficial, along with the regular practice of studying sacred scriptures this is called the austerity of speech.