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 it's first ever dip in iPhone sales from year to year -- Apple sold 51.2 million phones in its latest quarter, as compared to 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 over 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 worth more than $5 billion. "China could be a shadow for [the company], and we have to look at that," he told CNBC.