Bhagavad Gita - Chapter 4, Verse 7

Yadaa Yadaa hi dharmasya glaanirbhavati bhaarata |
Abhyuktaanamadarmasya tadaatmaanam srijaamyahamh ||

Meaning: When there is decay of dharma (righteousness) and rise of adharma (unrighteousness), then I (the Lord) am born in this world.