What is the Oldest Known Idiom?

| Sun Dec. 9, 2012 3:06 PM PST

I'm reading The Fine Print, by David Cay Johnston, right now, and early in the book he shares this anecdote about the treatment of debt in the Code of Hammurabi:

If you could not repay a debt because of circumstances beyond your control, such as a hailstorm flattening a field of grain, you could be excused from your debt—this amounted to an early form of bankruptcy protection. The clay tablet that recorded your debt could be washed, giving you a "clean slate," a term we use to this day to describe unpaid debts that are forgiven.

This got me wondering: what's the oldest source for an idiom still in common use? This one comes from about 1800 BC, so it's certainly in the running. Any other candidates?

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