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.
Often attributed to Robert Frost or Mark Twain.
Earliest reference for the quote was found in: The Battle of Home: Some Problems of Industrial Community (Bata Brands, 1944), p. 95: "The banks," wrote Thomas Bata Senior, "are institutions similar to those established for lending umbrellas. All of them try to lend you an umbrella when the sun is shining—and all of them run after you when it is beginning to rain."