“With a grain of salt.”

Liza Posas, Austry National Center librarian and archivist — quoted in: **Woo, Elaine (April 23, 2014). "Adrianne Wadewitz dies at 37; helped diversify Wikipedia" http://www.latimes.com/obituaries/la-me-adrianne-wadewitz-20140424,0,1077455.story. Los Angeles Times.
About

"The Salt of the Earth"
Source: The Harsh Voice: Four Short Novels (1935)

Life Without Principle (1863)
Context: I did not know that mankind were suffering for want of gold. I have seen a little of it. I know that it is very malleable, but not so malleable as wit. A grain of gold will gild a great surface, but not so much as a grain of wisdom.

"Zinc"
The Periodic Table (1975)

"View with a Grain of Sand"
Poems New and Collected (1998), The People on the Bridge (1986)
Context: We call it a grain of sand
but it calls itself neither grain nor sand.
It does just fine without a name,
whether general, particular,
permanent, passing,
incorrect or apt.

As quoted in Reader's Digest (April 1964)
Variant: I know a cure for everything. Salt water … in one form or another, sweat, tears or the salt sea.
Variant: The cure for anything is salt water — sweat, tears, or the sea.

Compare: "I give you sugar for sugar, but all you want is salt for salt/ Well if you can't get along with me, then it's your own fault." Richard Brown, James Alley Blues.
Song lyrics, The Basement Tapes (1975), Crash On The Levee (Down In The Flood) (recorded 1967)

This is a proverb which can be found in Robert Codrington's "Youth's Behaviour, Second Part" (1672) and in Thomas Fuller's "Gnomologia" (1732)
Misattributed

Sec. 57
Satyricon