Srimad Bhagavatam Canto 10, Chapter 48, Text 23

SB 10.48.23

tvayodito ’yam jagato hitaya
 yada yada veda-pathah puranah
badhyeta pasanda-pathair asadbhis
 tada bhavan sattva-gunam bibharti
 
Translation: 
 
You originally enunciated the ancient religious path of the Vedas for the benefit of the whole universe. Whenever that path becomes obstructed by wicked persons following the path of atheism, You assume one of Your incarnations, which are all in the transcendental mode of goodness.
Srimad Bhagavatam Canto 10, Chapter 48, Text 22
Srimad Bhagavatam Canto 10, Chapter 48, Text 24