Srimad Bhagavatam Canto-02, Chapter-07, Text-26
bhumeh suretara-varutha-vimarditayah
klesa-vyayaya kalaya sita-krsna-kesah
jatah karisyati jananupalaksya-margah
karmani catma-mahimopanibandhanani
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the world is overburdened by the fighting strength of kings who have no faith in God, the Lord, just to diminish the distress of the world, descends with His plenary portion. The Lord comes in His original form, with beautiful black hair. And just to expand His transcendental glories, He acts extraordinarily. No one can properly estimate how great He is.
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