I don't think we should confuse "idioms" with "proverbs". Sahl has given a very good example of a common idiom (I use it often myself). :)

A proverb is a nugget of wisdom, such as "Look before you leap", or its opposite "He who hesitates is lost".  Another one I recently heard from my Dad was "Make haste slowly", when he was advising me on a family situation.

Murphy's law: <em>If anything can go wrong, it will.</em>

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<em>A bird in the hand is worth two in the bush.</em>

<em>A penny saved is a penny earned.</em>

<em>An ounce of prevention is worth a pound of cure.</em>

Thank you @ Peachey for such info about the difference

it was the first time to know that :) 

Beat around the bush

for example: I'm not that kind of person who like to beat around the bush

it means I don't like to twist what I'm saying, I get to the point as possible as I can

P.S I think it is a very well known idiom in English 

Once a man, twice a child.