yadaa yadaa hi dharmasya glaanirbhavati bhaarata |
abhyuktaanamadarmasya tadaatmaanam srijaamyahamh ||
When there is decay of dharma (righteousness) and rise of adharma (unrighteousness),
then I (the Lord) am born in this world.
Bhagawat Gita Chapter 4, Verse 7