“A banker is a fellow who lends you his umbrella when the sun is shining, but wants it back the minute it begins to rain.”
According to R. Ken Rasmussen in The Quotable Mark Twain (1998), this is most probably not Twain's.
Misattributed
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Mark Twain637
American author and humorist 1835–1910Related quotes
“Don't pray when it rains if you don't pray when the sun shines.”
Satchel Paige (1906–1982) American baseball player and coach; Negro Leagues
New York Post (4 October 1959)

“If you do not want it to rain, always carry an umbrella.”
Andrzej Majewski (1966) Polish writer and photographer
Aphorisms. Magnum in Parvo (2000)
“[A banker is] a man who will lend you money if you can prove to him that you don't need it.”
Joe E. Lewis (1902–1971) American singer and comedian
Quoted by Leonard Lyons in his column https://secure.pqarchiver.com/washingtonpost_historical/doc/151726578.html, 15 October 1944
“When it rains, you put up an umbrella. That is the secret of success in business and management.”
Matsushita Konosuke (1894–1989) Japanese businessman
Kōnosuke Matsushita. Not for Bread Alone: A Business Ethos, a Management Ethic, 1984. p. 111