Legendary investor Warren Buffett told CNBC on Thursday that his conglomerate Berkshire Hathaway (NYSE: BRKa ) "bought $10 billion in US Treasurys last Monday." Buffett revealed they also bought $10 billion in Treasurys this Monday, shrugging off Fitch's U.S. credit rating downgrade. He added that "the only question for next Monday is whether we will buy $10 billion in 3-month or 6-month" T-bills. Buffett, speaking to CNBC's Becky Quick in an interview, said there are "some things people shouldn't worry about" and that "this is one." Fitch downgraded its long-term foreign currency issuer default rating for the U.S. to AA+ from AAA on Tuesday, based on the "expected fiscal deterioration over the next three years," as well as increasing debt. The move resulted in U.S. equities falling, but Buffett told CNBC that while the concerns are valid, it will not change his stance on U.S. Treasurys and the U.S. dollar, which is "the reserve currency of the world, and everybody knows it," he said.