Bhagavad Gita 4.8
To protect the righteous, to destroy the wicked, and to establish the kingdom of God, I am reborn from age to age.
परित्राणाय साधूनां विनाशाय च दुष्कृताम्।
धर्मसंस्थापनार्थाय संभवामि युगे युगे॥paritrāṇāya sādhūnāṃ vināśāya ca duṣkṛtām|
dharmasaṃsthāpanārthāya saṃbhavāmi yuge yuge||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).