Naik is facing a probe in India by the National Investigation Agency (NIA) and the Enforcement Directorate (ED) after his sermons on his Peace TV were cited as a reason by Bangladesh for an attack in Dhaka in 2016, which claimed 22 lives.
Mahathir, who was prime minister from 1981 to 2003 and is now the oldest elected leader in the world, remains a sharp political operator: many expect he will take steps to shore up Malay support for his government.
Muhammad Shafee is leading a defence team for Najib who faces charges of criminal breach of trust, money laundering and abuse of power at SRC International, a former 1MDB unit. Najib has denied any wrongdoing
Mahathir told Reuters in an interview that Malaysian investigators already "have an almost perfect case" against the principal suspects who had defrauded 1Malaysia Development Berhad (1MDB) and misappropriated billions of dollars in public funds.
In a late night Facebook post on Wednesday, Najib said Mahathir and his finance minister's "alarming and confusing" remarks about the country's debts and 1MDB liabilities "tell half the story" and had caused the stockmarket to fall.
The previous Malaysian administration had agreed in January to pay Houston-based Ocean Infinity up to $70 million if it found the plane during a search in the southern Indian Ocean that is expected to end in June.