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.