17:15  

अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत् ।
स्वाध्यायाभ्यसनं चैव वाङ्मयं तप उच्यते ॥१७- १५॥

anudwegakaraM waakyaM satyaM priyahitaM cha yat ।

swaadhyaayaabhyasanaM chaiwa waa~NmayaM tapa uchyate ॥१७- १५॥

Source: Automatic ITRANS Transliteration

Those undivisive words, truthful and pleasing,
Self-reflective and studious are called austerity of speech.

Source: Milind S. Pandit

Speech that is not offensive, truthful, pleasant, beneficial, and is used for the regular reading of scriptures is called the austerity of word.

Source: The Bhagavad-Gita: The Sacred Song

Austerity of speech consists in speaking truthfully and beneficially and in avoiding speech that offends. One should also recite the Vedas regularly.

Source: Bhagavad Gita As It Is