SB 11.5.20
sri-karabhajana uvaca
krtam treta dvaparam ca
kalir ity esu kesavah
nana-varnabhidhakaro
nanaiva vidhinejyate
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sri Karabhajana replied: In each of the four yugas, or ages — Krta, Treta, Dvapara and Kali — Lord Kesava appears with various complexions, names and forms and is thus worshiped by various processes.