I may be cheating, as this is not a quote by a famous singer, but by his grandma, and it is not about singing, but about how he talks. Nonetheless, I find the anecdote amusing.
In his memoirs, Hans Hotter tells the story that , when he was young, he went to Berlin to study opera singing. When he went home to take a break, his grandmother was shocked by his newly acquired accent: “You sound like a Prussian!”