John Mayall and the Bluesbreakers' rocking 1966 cover of Ray Charles' What'd I Say features a very young Eric Clapton on lead guitar.
However, to put it bluntly, even though it appears on a legendary guitar album – Blues Breakers with Eric Clapton – What'd I Say is not a standout track by any means. It just sort of sits there, and its lengthy (and weird) drum solo by Hughie Flint isn't exactly Moby Dick. Who knows – maybe it was a crowd favorite at the Bluesbreakers' live shows.
Anyway, there is this oddity to consider: When the rest of the band comes back into the song after Flint's drum solo (at 3:36 in the top video), Clapton is playing the very-hard-to-miss guitar riff from the Beatles' late-1965 single Day Tripper – over and over again. Blues Breakers was recorded in March 1966, so there's no question as to whose riff it was. George Harrison had played it first.
Plagiarism? Maybe by today's litigation-happy standards. But in reality, it's just a 21-year-old (almost 22-year-old) guitarist being irreverent and having fun, quoting a famous song within another song, just as he did when he quoted the melody to Rodgers & Hart's Blue Moon in the guitar solo to Cream's Sunshine of Your Love a year later (2:04 in the bottom video).
Clapton had already met the Beatles at this point (they first met in December 1964 when Clapton was in the Yardbirds), and his decades-long friendship with Harrison was in its very early stages. Let's consider this little What'd I Say/Day Tripper episode the first recorded connection between Clapton and the Beatles.
Clapton's "Day Tripper" riff starts at 3:36 in the top video.
