Bhagavad Gita 4.8

परित्राणाय साधूनां विनाशाय च दुष्कृताम् |
धर्मसं स्थापनार्थाय संभावनामि युगे युगे

Translation

To protect the righteous, to annihilate the wicked, and to reestablish the principles of dharma I appear on this earth, age after age.