When a billionaire investment manager and a Nobel-Prize winning economist are sounding the alarm over the speculative fever in digital currencies, it may be prudent to review the sage old wisdom on the topic from the Oracle of Omaha himself.

Warren Buffett was specifically asked for his views on bitcoin several years ago.

“Stay away from it. It’s a mirage basically. It’s a method of transmitting money. It’s a very effective way of transmitting money and you can do it anonymously and all that. A check is a way of transmitting money too. Are checks worth a whole lot of money? Just because they can transmit money?” Buffett said on CNBC in 2014. “I hope bitcoin becomes a better way to do it. But you can replicate it a bunch of different ways. The idea that it [bitcoin] has some huge intrinsic value is just a joke in my view.”

Digital currency advocates will point out that the price of bitcoin has risen more than sevenfold since Buffett expressed his negative view. In addition, the cryptocurrency was up nearly 380 percent this year through Thursday morning, according to data from industry website CoinDesk.

But the famed investor never said he can predict exactly when bubbles peak, just that the feverish times will end some day. He specifically warned about the perils of “effortless money” through speculation during the dot-com bubble time period:

“The line separating investment and speculation, which is never bright and clear, becomes blurred still further when most market participants have recently enjoyed triumphs. Nothing sedates rationality like large doses of effortless money … But a pin lies in wait for every bubble. And when the two eventually meet, a new wave of investors learns some very old lessons: First, many in Wall Street – a community in which quality control is not prized – will sell investors anything they will buy. Second, speculation is most dangerous when it looks easiest.” – Warren Buffett, Berkshire Hathaway 2000 shareholder letter

In similar fashion, billionaire investor Howard Marks told his clients to avoid high-flying digital currencies in July.

“In my view, digital currencies are nothing but an unfounded fad (or perhaps even a pyramid scheme), based on a willingness to ascribe value to something that has little or none beyond what people will pay for it,” Marks wrote in an investor letter.

The manager then compared cryptocurrencies to the tulip mania of 1637, the South Sea bubble of 1720 and the internet bubble of 1999.

To be sure, there is no way to know we are near the top even if digital currencies are similar to previous asset bubbles. However Buffett has proven to be rarely wrong over the long run and investors should be cognizant of his warnings on bitcoin.