From his 2013 letter to shareholders (pg20)
http://www.berkshirehathaway.com/letters/2013ltr.pdf
"My money, I should add, is where my mouth is: What I advise here is essentially identical to certain
instructions I’ve laid out in my will. One bequest provides that cash will be delivered to a trustee for my wife’s
benefit. (I have to use cash for individual bequests, because all of my Berkshire shares will be fully distributed to
certain philanthropic organizations over the ten years following the closing of my estate.) My advice to the trustee
could not be more simple: Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P
500 index fund. (I suggest Vanguard’s.) I believe the trust’s long-term results from this policy will be superior to
those attained by most investors – whether pension funds, institutions or individuals – who employ high-fee
managers."