Bhagavad Gita Chapter 4.8
paritrāṇāya sādhūnāḿ
vināśāya ca duṣkṛtām
dharma-saḿsthāpanārthāya
sambhavāmi yuge yuge
Translation
To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.