Shri Krishna says in the Bhagavad Geeta:

 

यदा यदा हि धर्मस्य ग्लानिर्भवति भारत | अभ्युत्थानमधर्मस्य तदात्मानं स्रुजाम्याहं || (४ : ७ )

Yada yada hi dharmasya glanirbhavati bharat| Abhyutthanamadharmasya tadatmanam srujamyaham|| (4: 7)

परित्राणाय साधूनां विनाशाय  च  दुष्कृताम | धर्मसंस्थापनार्थाय  संभवामि  युगे  युगे || (४ :८ )

Paritraanaaya sadhunam vinashaya cha dushkrutam|

Dharmasamsthapanarthaya sambhavami yuge yuge|| (4: 8)

Whenever there is a decline in righteousness, O Bharata, and a rise of unrighteousness, then I manifest myself. For the protection of the good and for the destruction of the wicked. For the establishment of righteousness, I am born in every age, from age to age.