Bhagwad Gita
Chapter 17, Verse 15
अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत्।स्वाध्यायाभ्यसनं चैव वाङ्मयं तप उच्यते।।17.15।।
Austerity of speech consists in speaking truthfully and beneficially and in avoiding speech that offends. One should also recite the Vedas regularly.