I seem to recall Ueshiba himself saying it in the audio interview from Aikido Journal, but I'd have to double check to be certain. I was just curious if you might know of earlier examples of him saying similar things or even if there's anything similar going back as far as Takeda? I know Takeda said Daito-ryu was also an art of peace or something similar, but it's always seemed to be more of a response to the recognition Ueshiba was getting than a way of promoting what he was doing. While Takeda seemed to have little problem teaching anyone who had the money, I don't think that's quite the same as "aikido is for everyone", but maybe I'm wrong there.
Takeda would teach anybody - but we woudn't teach everybody everything.
Morihei definitely jumped on the bandwagon at some point (at least publicly), but that seems to have come after some resistance.