Najib is appealing a lower court decision in July that struck out his request for a judicial review to compel the government to confirm the existence of and execute a royal order entitling him to serve the remainder of his jail term at home.
Najib maintains that an "addendum order" was issued by Malaysia's former king alongside a pardons board decision made public in February to halve his prison sentence for graft in the multibillion-dollar 1MDB scandal from 12 years to six.
It is unclear if the document exists and there has been no comment from former king Al-Sultan Abdullah, whose reign ended in January. Malaysia has a unique system of monarchy in which the country's nine sultans take turns to be king every five years. The king chairs the pardons board.
Najib's lawyer, Muhammad Shafee Abdullah, said the ex-premier's son filed an affidavit on Tuesday affirming he had received a copy of the addendum from Al-Sultan Abdullah's royal household in Pahang state.
Muhammad Shafee declined to disclose the contents of the document citing a protective order, but said it confirmed instructions for Najib to serve his sentence at home.
"(This) additional and fresh evidence is to prove conclusively that it exists," he told reporters after Thursday's hearing.
The Court of Appeal had allowed proceedings to be postponed to a later date to allow prosecutors' time to review the affidavit.
Najib was jailed in 2022, two years after being found guilty of criminal breach of trust and abuse of power for illegally receiving funds misappropriated from a unit of state fund 1Malaysia Development Berhad (1MDB).
Malaysian and U.S. investigators estimate $4.5 billion was stolen from 1MDB and more than $1 billion channelled to accounts linked to Najib.
In the February statement, the pardons board said Najib was now expected to be released in August 2028. It also reduced the ex-premier's fines, sparking uproar in Malaysia.
The government last month said it would introduce a law next year allowing house arrest for some offences, but denied it was aimed at getting Najib or other politicians accused of corruption out of jail.
