One verse

Srimad Bhagavatam 8.14.4

8.14.4

चतुर्युगान्ते कालेन ग्रस्ताञ्छ्रुतिगणान्यथा । तपसा ऋषयोऽपश्यन्यतो धर्म: सनातन: ॥ ४ ॥

catur-yugante kalena grastan chruti-ganan yatha tapasa rsayo ’pasyan yato dharmah sanatanah

catuh-yuga-ante
at the end of every four yugas (Satya, Dvapara, Treta and Kali)
kalena
in due course of time
grastan
lost
sruti-ganan
the Vedic instruction
yatha
as
tapasa
by austerity
rsayah
great saintly persons
apasyan
by seeing misuse
yatah
wherefrom
dharmah
occupational duties
sanatanah
eternal.

At the end of every four yugas, the great saintly persons, upon seeing that the eternal occupational duties of mankind have been misused, reestablish the principles of religion.