Legends sources:
The Complete Star Wars Encyclopedia
The New Essential Chronology
The Official Starships and Vehicles Collection
Jedi Battles
Obi-Wan and Vader duel on Mustafar while Yoda and Palpatine duel on Coruscant. Palpatine is too strong for the aging Yoda, who escapes with Bail Organa.
The Comics Companion
(apologies for the lack of a scan)
Finally, he saw the truth. This truth: that he, the avatar of light, Supreme Master of the Jedi Order, the fiercest, most implacable, most devastatingly powerful foe the darkness had ever known...
just-
didn't-
have it.
Revenge of the Sith
Even the greatest warriors of the Jedi - Mace Windu, Kit Fisto, even Yoda - could not defeat the Sith.
The Last of the Jedi: Underworld
George Lucas also has given his verdict:
Insider #86