Mahathir's focus is also to tackle corruption scandals that have dogged Najib's administration.
Earlier this week, Najib's long-ruling Barisan Nasional coalition suffered a shock electoral defeat.
Speaking at a news conference, Dr Mahathir said he would announce a 10-member cabinet on Saturday, including ministers for finance, defence and home affairs.
Police has stepped up security at the entrance to the private residence of former Prime Minister Datuk Seri Najib Razak at Jalan Langgak Duta.
The Immigration Department, which earlier denied that Najib and his wife were barred from leaving, says in a brief statement Saturday the duo have "just been blacklisted from leaving the country".
Fernando Alonso and McLaren F1 happy with qualifying results in Barcelona
The Spaniard says he will not decide on his plans for 2019 until after F1's August summer break. We just need new parts as soon as possible and to keep going in this direction.
Mahathir and Anwar reconciled in recent years in a bid to oust Najib, an extraordinary twist even by the head-spinning standards of Malaysian politics.
A few minutes later Najib posted on Twitter that he respects the direction by the Immigration Department and would remain in the country and has cancelled his overseas trip.
"It is true that I prevented Najib from leaving the country" Mahathir told reporters on Saturday.
"It's been cleared, there's been no wrongdoing - I stand by it, " Najib said in an interview last month.
Joe Biden is furious over Trump aide's John McCain comment
Congressman Adam Kinzinger, a Republican and former US Air Force pilot in Iraq and Afghanistan, blasted Sadler . The White House's position on this kind of situation has always come down to not giving an inch.
Malaysia's new finance minister is seen as a safe pair of hands as he takes on the huge task of bringing down debt while scrapping a consumption tax that was key to Prime Minister Mahathir Mohamad's unexpected election win last week.
"It is a very complex thing because it involves a lot of people, it involves a lot of decisions made, and the money you know has to be investigated as to the money laundering", he said.
The National Front, which has ruled Malaysia since independence from Britain, obtained only 79 of the 222 parliamentary seats, losing power to Mahathir Mohamad's four-party alliance amid public anger over the 1MDB scandal and rising cost of living.
Possible return of Anwar Ibrahim: Mahatir said after his victory over Najib that the country's monarch had agreed to pardon jailed opposition politician Anwar Ibrahim, who has been serving a jail sentence since 2015 on "sodomy" charges that Anwar says were politically motivated.
U.S. Department of Justice documents allege that $681 million from 1MDB was transferred to the personal account of a person identified as Malaysian Official One, which U.S. and Malaysian sources have confirmed was Najib.
Salmonella Outbreak Tied to Egg Recall Sickens 12 More
In 1990, the egg producer was linked to three separate salmonella outbreaks that sickened hundreds of people, the paper reported. Eleven of the 35 people who got sick this time around had to be hospitalized, and so far no deaths have been reported.
He denied any wrongdoing, even as US authorities alleged that over $4.5 billion was stolen from the fund in a fraud orchestrated by a financier known to be close to Najib and his family.