Warren Buffett's Berkshire Hathaway Inc. (BRK.A) bought 75 million more shares of Apple Inc. (AAPL) in the first quarter of 2018, bringing the total investment to 240.3 million shares worth $42.5 billion.

Buffett, who has called the iPhone "a very sticky product" in the past, revealed the stake purchase to CNBC on Thursday night. "It is an unbelievable company," he said. "If you look at Apple, I think it earns almost twice as much as the second most profitable company in the United States." (See also: 6 Stocks Warren Buffett Might Buy Next)

Apple stock was up half a percent in pre-market trading.

Two years ago, Berkshire surprised investors when it disclosed its first investment of 9.8 million shares in the tech giant. In February, Buffett said the stake grew by about 23 percent since the end of September to roughly 165.3 million shares worth $28 billion at the end of 2017. Some are speculating that Berkshire's $116 billion in cash and equivalents is the reason for the latest investment. It now owns a 4.7% stake in Apple. "Our smiles will broaden when we have redeployed Berkshire's excess funds into more productive assets," the Oracle of Omaha wrote in his annual shareholder letter in February.

On Tuesday, Apple proved Wall Street naysayers wrong when it released its quarterly earnings report. For weeks prior analysts had warned that iPhone demand is flagging based on reports from a few suppliers. "The idea that you're going to spend loads of time trying to guess how many iPhone X ... are going to be sold in a 3 month period totally misses the point," Buffett said in an interview with CNBC on Friday. "It's like worrying about the number of BlackBerrys 10 years ago." (See also: Morgan Stanley: We Were Wrong, Apple to Gain 13%)

The legendary investor has said he was afraid of tech stocks in the past because he did not understand the business models and he regrets not buying Alphabet Inc. (GOOG) and Amazon.com Inc. (AMZN) shares years ago. The flip phone user believes Apple products have a psychological hold on users, and he views it as a consumer company, according to CNBC.

Berkshire will release its first quarter earnings report this weekend as thousands of shareholders will gather in Omaha for the annual shareholders meeting, which is also known as "Woodstock for Capitalists."