Warren Buffett has earned a reputation as one of Wall Street’s greatest investors. The many acquisitions and stock purchases he has engineered since assuming control of Berkshire Hathaway in 1965 have created immense wealth for shareholders. Berkshire stock has returned 20% annually over the last six decades, while the S&P 500 (SNPINDEX: ^GSPC) has gained 10.4% annually.

Nevertheless, I suspect many investors ignore one of his more prudent recommendations. Buffett has often advised investors to periodically buy and patiently hold an S&P 500 index fund. “I recommend the S&P 500 index fund, and have for a long, long time, to people,” he said in 2021.

Here’s how that advice could turn a $400 monthly investment into $851,800 over 30 years. The Vanguard S&P 500 ETF provides exposure to influential companies like Apple, Nvidia, and Tesla
The S&P 500 measures the performance of 500 large and profitable U.S. companies that cover about 80% of domestic equities and 50% of global equities by market value. The Vanguard S&P 500 ETF (NYSEMKT: VOO) tracks the S&P 500, allowing investors to spread capital across many of the most influential businesses in the world.

Buffett added context in his 2016 shareholder letter. “American business — and consequently a basket of stocks — is virtually certain to be worth far more in the years ahead.” An S&P 500 index fund satisfies that criterion. It is a readymade portfolio comprising hundreds of companies that form the foundation of the United States economy, which itself is the world’s most prosperous and innovative large economy, according to JPMorgan Chase CEO Jamie Dimon.