CC Ādi 3.23

paritrāṇāya sādhūnāṁ
vināśāya ca duṣkṛtām
dharma-saṁsthāpanārthāya
sambhavāmi yuge yuge
Synonyms: 
paritrāṇāya — for the deliverance; sādhūnām — of the devotees; vināśāya — for the destruction; ca — and; duṣkṛtām — of the miscreants; dharma — religious principles; saṁsthāpana-arthāya — for the purpose of establishing; sambhavāmi — I appear; yuge yuge — in every age.
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.’
Purport: 

texts 22 and 23 were spoken by Lord Kṛṣṇa in the Bhagavad- gītā (4.7-8). texts 24 and 25, which follow, are also to be found in the Bhagavad-gītā (3.24, 21).