The stake represents just about 0.1 per cent of Amazon's outstanding equity, the report said.
Buffett first disclosed the new investment in Amazon to CNBC earlier this month.
He said that someone at Berkshire's asset management desk just invested in some Amazon shares but "it wasn't me."
"One of the fellows in the office that manage money ... bought some Amazon," Buffett was quoted as saying.
"The truth is that I've watched Amazon from the start and I think what Jeff Bezos has done is something close to a miracle, and the problem is if I think something is going to be a miracle I tend not to bet on it," Buffett said.
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