Bhagavad Gita, Chapter 4, Verse 7

Whenever, O descendant of Bharata, there is decline of Dharma, and rise of Adharma, then I embody Myself.

When right living declines and wrong takes over, Krishna says he steps in.

This is one of the Gita's best-known lines because it names a pattern rather than a one-time event. Whenever balance breaks down, something steps in to restore it, and that's framed as a recurring promise, not a single historical moment.