If people knew how hard I work to get my mastery, it wouldn’t seem too wonderful after all.
Michelangelo (1475-1564) Italian artist, architect, poet [Michelangelo di Lodovico Buonarroti Simoni]
(Attributed)
The earliest attributions only go back to the Twentieth Century (e.g., 1929) in non-academic contexts. No original source is known.
A related attribution, regarding the Sistine Chapel -- "If you knew how much work went into it, you would not call it genius." -- only can be found in the Twenty-First century (e.g., August 2001).