Bhagavad Gita 17.15

अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत् |
स्वाध्यायाभ्यसनं चैव वाङमयं तप उच्यते

Translation

Words that do not cause distress, are truthful, inoffensive, and beneficial, as well as regular recitation of the Vedic scriptures—these are declared as austerity of speech.