Bhagavad Gita : अध्याय 17 का श्लोक 15
अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत्।स्वाध्यायाभ्यसनं चैव वाङ्मयं तप उच्यते॥
भावार्थ : जो उद्वेग न करने वाला, प्रिय और हितकारक एवं यथार्थ भाषण है (मन और इन्द्रियों द्वारा जैसा अनुभव किया हो, ठीक वैसा ही कहने का नाम 'यथार्थ भाषण' है।) तथा जो वेद-शास्त्रों के पठन का एवं परमेश्वर के नाम-जप का अभ्यास है- वही वाणी-सम्बन्धी तप कहा जाता है ॥15॥
Austerity of speech consists in speaking truthfully and beneficially and in avoiding speech that offends. One should also recite the Vedas regularly.
One should not speak in such a way as to agitate the minds of others. Of course, when a teacher speaks, he can speak the truth for the instruction of his students, but such a teacher should not speak to others who are not his students if he will agitate their minds. This is penance as far as talking is concerned. Besides that, one should not talk nonsense. When speaking in spiritual circles, one's statements must be upheld by the scriptures. One should at once quote from scriptural authority to back up what he is saying. At the same time, such talk should be very pleasurable to the ear. By such discussions, one may derive the highest benefit and elevate human society. There is a limitless stock of Vedic literature, and one should study this. This is called penance of speech.