Najib Abdul Razak is citing sitting premier Anwar Ibrahim’s speech at the recent PKR special convention nearly two months ago to strengthen his legal challenge for the implementation of an alleged royal addendum to allow him to serve the remainder of his six-year jail sentence as house arrest.

In his affidavit filed at the Kuala Lumpur High Court on May 25, the former prime minister alleged that the PKR president’s speech at the party convention on April 21 confirmed the existence of the purported supplementary royal decree.

On that date, Anwar reportedly said...