Bhagwad Gita
Chapter 4, Verse 8
परित्राणाय साधूनां विनाशाय च दुष्कृताम्।
धर्मसंस्थापनार्थाय संभवामि युगे युगे।।4.8।।
In order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I advent Myself millennium after millennium.