Former President Barack Obama warned President Donald Trump against hiring Mike Flynn as national security adviser, NBC News reported Monday. NBC's report, citing three former Obama administration officials, comes the same day that former acting Attorney General Sally Yates is expected to testify that Flynn misled the Trump White House about his contacts with Russia's ambassador to the U.S. Trump disparaged Yates on Twitter on Monday morning, encouraging lawmakers to ask her if she knows how classified information got "into the newspapers" after she explained it to White House counsel.