The Malaysian Anti-Corruption Commission (MACC) did not reveal their findings or say whether any wrongdoing was involved. It would be up to Attorney-General Mohamed Apandi Ali to decide any further action, it said.
Najib has weathered months of calls to resign over alleged financial mismanagement at indebted state fund 1Malaysia Development Berhad (1MDB), whose advisory board he chairs.
Controversy turned to scandal in July when the Wall Street Journal reported that investigators looking into 1MDB had identified funds worth 2.6bn ringgit ($605.36mn) that had been transferred directly to the prime minister’s bank accounts.
Najib has denied any wrongdoing and says he did not take any money for personal gain.
The MACC had earlier said the funds were a political donation from an unidentified Middle Eastern benefactor.
The scandal has shaken investors in Southeast Asia’s third-biggest economy and rocked public confidence in the coalition led by Najib’s United Malays National Organisation (UMNO) party, which has held power since independence in 1957.
The fund had racked up more than $11bn in debt - a burden that had weighed on the ringgit currency - before beginning a restructuring programme this year.
“The MACC has high hopes that the Attorney-General in his capacity as the public prosecutor will make a professional decision,” the MACC’s deputy chief commissioner of operations, Mohd Shukri Abdull, said in a statement yesterday.
Najib said in a New Year’s message that a string of asset sales combined with other deals would see 1MDB’s debt cut by about 40.4bn ringgit ($9.4bn).
“It is therefore clear that 1MDB’s major challenges are now behind it,” he said.
The MACC submitted a separate report on SRC International, a former 1MDB subsidiary being probed for an alleged misappropriation of funds worth 4bn ringgit ($931.32mn).
The WSJ report, which Reuters has not independently verified, had also said that 42mn ringgit ($9.78mn) of the funds channeled into Najib’s account originated from SRC.
1MDB is under investigation by law enforcement agencies in Switzerland, Hong Kong and the United States, media and sources have said.
Opposition leaders and some establishment figures have called for Najib to step down over the scandal.
However, Najib  still enjoys the backing of most of UMNO’s powerful division chiefs, and even his fiercest internal critics, such as influential former Prime Minister Mahathir Mohamad, accept that he cannot be unseated.