Bhagavad Gita · 17.15

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

anudvega-karaṁ vākyaṁ satyaṁ priya-hitaṁ cha yat svādhyāyābhyasanaṁ chaiva vāṅmayaṁ tapa uchyate

Speech that causes no agitation, that is truthful, pleasant, and beneficial, as well as the practice of studying the scriptures — this is called austerity of speech.

— Bhagavad Gita 17.15

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)