Bhagavad Gita · 4.8

परित्राणाय साधूनां विनाशाय च दुष्कृताम्।धर्मसंस्थापनार्थाय सम्भवामि युगे युगे॥

paritrāṇāya sādhūnāṁ vināśhāya cha duṣhkṛitām dharma-saṁsthāpanārthāya sambhavāmi yuge yuge

For the protection of the righteous, the destruction of the wicked, and the establishment of dharma, I appear in every age.

— Bhagavad Gita 4.8

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)