यदा यदा हि धर्मस्य ग्लानिर्भवति भारत | अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् || 4.7 ||  Whenever there is a decline in righteousness and an increase in unrighteousness, O Arjun, at that time I manifest myself on earth.  

परित्राणाय साधूनां विनाशाय च दुष्कृताम् | धर्मसंस्थापनार्थाय सम्भवामि युगे युगे || 4.8 ||  To protect the righteous, to annihilate the wicked, and to reestablish the principles of dharma I appear on this earth, age after age.  

-Lord Krishna [8th Avatar Of Triune Deity Vishnu], Bhagavad Gita || 4.7-8 ||