Bhagavad Gita (Chapter IV-7)

Shloka :

'Yada Yada Hi Dharmasya

Glanirva Bhavathi Bharatha,
Abhyuthanam Adharmaysya
Tadatmanam Srijami Aham'.

Meaning :

"Whenever there is decay of

 righteousness O! Bharatha

And a rise of unrighteousness

then I manifest Myself!".