Bhagavad Gita 17.15अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत्।स्वाध्यायाभ्यसनं चैव वाङ्मयं तप उच्यते ॥

The words of the man who practises the austerity of speech cannot cause pain to others. His words will bring cheer and solace to others. His words prove beneficial to all. The organ of speech causes great distraction of mind. Control of speech is a difficult discipline but you will have to practise it if you want to attain supreme peace. Nothing is impossible for a man who has a firm determination, sincerity of purpose, iron will, patience and perseverance.It is said in Manu Smritiसत्यं ब्रूयात् प्रियं ब्रूयात् न ब्रूयात् सत्यमप्रियम्।प्रियं च नानृतं ब्रूयात् एष धर्मः सनातनः।।One should speak what is true one should speak what is pleasant. One should not speak what is true if it is not pleasant nor what is pleasant if it is false. This is the ancient Dharma.Excitement Pain to living beings.Speech, to be an austerity, must form an invariable combination of all the four attributes mentioned in this verse, viz., nonexciting or nonpainful, truthful, pleasant and beneficial if it is wanting in one or the other of these attributes, it cannot form the austerity of speech. Speech may be pleasant but it it is lacking in the other three attributes, it will no longer be an austerity of speech.