Bhagavad Gita Chapter 17 Verse 19

Gita 17.19

मूढग्राहेण, आत्मनः, यत्, पीडया, क्रियते, तपः,
परस्य, उत्सादनार्थम्, वा, तत्, तामसम्, उदाहृतम्।।19।।

Gita 17.19

Moodgraahen, aatmanH, yat’, peedya, kriyate, tapH,
Parasya, utsaadnaarthm’, va, tat’, taamsam’, udaahrtam’ ||19||

Translation: (Yat’) which (tapH) austerity (moodgraahen) foolishly with stubbornness (aatmanH) of mind, speech and body (peedya) inflicting pain to (va) or (parasya) of others (utsaadnaarthm’) to cause harm (kriyate) is practised (tat’) that austerity (taamsam’) Tamas (udaahrtam’) is said to be. (19)

Translation: The austerity which is practised foolishly with stubbornness by inflicting pain to mind, speech and body, or to cause harm to others, that austerity is said to be Tamas. {Its evidence is also in Gita Gita Chapter 3 Verse 6.}