Why billionaire Carl Icahn just dumped every share he has in Apple

Apple has had a tough week after reporting that its quarterly revenue fell for the first time since 2003. Things got worse Thursday when billionaire investor Carl Icahn announced that he had sold his shares in the company.

The news sent Apple stock, already depressed by the disappointing earnings report earlier this week, down another 3 percent. Apple's stock, one of the most widely held in the world, is now down about 10 percent this week, erasing about $56 billion from its market value.

Icahn has been one of Apple's most prominent — and vocal — investors. In 2014, he suggested that Apple was undervalued and was worth more than $1 trillion. Icahn has also repeatedly tussled with the firm about a program for buying back its stock, which could raise its value. The company eventually relented.

But, Icahn said on CNBC Thursday afternoon, he has now sold all of his shares in the company and made a $2 billion profit.

"We obviously made a great deal of money," Icahn said.

Apple did not immediately respond to a request for comment.

Apple is a "great company," Icahn said. Icahn said he called Tim Cook, Apple's chief executive, to alert him to the news. "He seemed sort of sad to hear that," Icahn said.

Apple reported its earnings earlier this week, revealing that its revenue dropped from the previous year for the first time in 13 years. It also reported its first ever decline in iPhone sales from year to year — Apple sold 51.2 million phones in its latest quarter, down from 61.2 million the previous year.

The report ended a remarkable run that helped the tech firm become the world's most valuable company.

In Apple's latest earnings call,  Cook called the current situation a "pause" in Apple's growth. Yet with a majority of its revenue wrapped up in the slowing smartphone market, any perception that the iPhone is weakening has enormous repercussions.

Apple, like every other smartphone vendor, has watched consumer appetites for smartphones shrink during the past several quarters. That slowing growth seems to have caught up with the company this quarter, particularly in the critical market of China. Apple reported that revenue was down 26 percent from the same period last year, making it the region of the world where the firm saw the greatest downturn.

That appears to be behind Icahn's decision to sell his shares in the company, which were once valued at more than $5 billion. "China could be a shadow for [the company], and we have to look at that," he told CNBC.