Here is an excerpt from a memo from Senator Chuck Graslley that details the retraction. Via Washington Examiner:
The Justice Department has retracted a second statement made to the Senate Judiciary Committee. During a hearing last week, Attorney General Eric Holder claimed that his predecessor, then-Attorney General Michael Mukasey, had been briefed about gunwalking in Operation Wide Receiver. Now, the Department is retracting that statement and claiming Holder "inadvertently" made that claim to the Committee. The Department's letter failed to apologize to former Attorney General Mukasey for the false accusation. This is the second major retraction the Justice Department has made in the last seven months. In December 2011, the Department retracted its claim that the ATF had not allowed illegally purchased guns to be trafficked to Mexico.Here is the exchange between Cornyn and Holder in which the Attorney General made the original charge that he has since retracted.