“If you want to find the secrets of the universe, think in terms of energy, frequency and vibration.”
First attibution is to Ralph Bergstresser who claims to have heard this from Tesla in a conversation "following an experience with the Maharaja's son".
Disputed
Source: Coments From The Inventor of the Purple Harmony Plates http://www.bibliotecapleyades.net/ciencia/esp_ciencia_universalenergy02.htm,
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Nikola Tesla 125
Serbian American inventor 1856–1943Related quotes

There's no evidence that Einstein ever said this. (Source: http://quoteinvestigator.com/2012/05/16/everything-energy/.)
Misattributed

[Quantum Mechanics, 1992, Princeton University Press, 11, https://books.google.com/books?id=hTaw4Ht4Vi8C&pg=PA11]

Source: Spiritual Journey: Michio Kushi's Guide to Endless Self-Realization and Freedom (1994, with Edward Esko), p. 54