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Warren Buffett has added around $6 billion to Berkshire Hathaway's sizable cash pile this summer with a string of Bank of America stock sales since mid-July.

According to a new filing Friday, sales of 21.1 million shares on Wednesday, Thursday, and Friday generated $848.2 million, which is an average price of $40.24.

Berkshire has sold Bank of America shares for six straight sessions. Since it began to reduce its holding on July 17, it has sold shares during 21 of the past 33 sessions.

In total, Berkshire has cut its BofA holding by 14.5% with the sale of 150.1 million shares for $6.2 billion. That's an average of $41.33 per share.

BofA is Berkshire's third largest equity holding, accounting for around 11% of its portfolio.

Berkshire remains Bank of America's largest shareholder with a 11.4% stake of 882.7 million shares valued at nearly $36 billion.

As the sales continue, however, it is approaching the Vanguard Group's 639 million shares.

While there are several theories on why Berkshire is selling a stock that Buffett said as recently as last year he didn't want to sell, despite his concerns about the banking sector overall, there's been no explanation from Omaha so far.

Buffett turned 94 on Friday. As of June 30, Berkshire's cash pile stood at a record $277 billion.