Bhagavad Gita 17.15
अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत्। स्वाध्यायाभ्यसनं चैव वाङ्मयं तप उच्यते।।17.15।।
anudvegakaraṃ vākyaṃ satyaṃ priyahitaṃ ca yat| svādhyāyābhyasanaṃ caiva vāṅmayaṃ tapa ucyate||17.15||
anudvegakaram vakyam satyam priyahitam ca yat svadhyayabhyasanam caiva vanmayam tapa ucyate17.15
Meaning
Speech that causes no agitation, that is truthful, pleasant, and beneficial, and also the practice of self-study — these constitute the austerity of speech.
जो वाणी किसी को उद्वेग न दे, सत्य हो, प्रिय हो, और हितकारी हो, तथा स्वाध्याय का अभ्यास हो; इसे वाचिक तप कहा जाता है।
This is verse 15 of 28 in Chapter 17 (Shraddhatraya Vibhaga Yoga) of the Bhagavad Gita.