Sessions writes that it was "an incredible honor to work with people who day after day and year after year consistently exceeded expectations".
"It is time for Rosenstein, who is the acting attorney general for the purposes of this investigation, to order Mueller to limit the scope of his investigation to the four corners of the order appointing him special counsel". Matthew Whitaker is now acting attorney general, and he's now in position to oversee the Russian Federation investigation led by Special Counsel Robert Mueller.
Protesters are calling for Acting Attorney General Matthew Whitaker to recuse himself from overseeing the investigation, handing the reins back to Deputy Attorney General Rod Rosenstein, according to the statement.
The two people close to Whitaker also said they strongly believe he would not approve any request from special counsel Robert S. Mueller III to subpoena the president.
Trump has gone after the Justice Department and Sessions over the ongoing investigation into ties between the Trump election campaign and Russian Federation by special counsel Robert Mueller.
The move left unclear the fate of Deputy Attorney-General Rod Rosenstein, who appointed Mueller and has overseen his probe from the beginning.
"It is impossible to read Attorney-General Sessions' firing as anything other than another blatant attempt by (Mr Trump) to undermine and end Special Counsel Mueller's investigation", House minority leader Nancy Pelosi wrote on Twitter.
A Kremlin spokesman, Dmitry Peskov, on Thursday called Mueller's investigation a "headache" for USA authorities but said it "has nothing to do with us", and he declined to comment on Sessions' departure.
Matt Whitaker has been named acting US attorney general following the resignation of Attorney General Jeff Sessions. When Sessions's previous chief of staff, Jody Hunt, was departing, Sessions brought Whitaker in for an interview and came to like him, the person said. He was a George W. Bush loyalist - named a former U.S. attorney in Iowa under Bush - but like many others, he has become a big admirer of Trump along the way. "And that attorney-general doesn't fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to nearly a halt", said Mr Whitaker in an interview with CNN a year ago.
Many voiced concerns Wednesday that Whitaker may have potential conflicts of interest, especially in light of his attacks on the investigation.
Gonzales, who was the attorney general in George W. Bush's administration from 2005 to 2007, said he doesn't know Whitaker, his qualifications or motivations, but there's a lot that calls into question his appointment.