Then on 3 July, Mr Najib was arrested by anti-corruption authorities. Najib's laywer Muhammad Shafee Abdullah protested the comment calling it "nonsense" and "coffeeshop talk".
"I am confident of my innocence, I believe in my innocence and this is the best chance to clear my name", Najib told reporters outside the courtroom after being released on bail.
"As a normal human being, I am not ideal but believe me that the accusations against me and my family are not all true", he said.
Malaysian police say they have seized almost $275 million worth of jewelry, handbags, watches and cash from premises linked to Najib, while authorities have frozen more than 400 bank accounts as part of the 1MDB probe.
Mahathir, 92, said in an interview with Reuters last month that embezzlement and bribery with government money were among the charges that Malaysia was looking to bring against Najib, 64, adding they had "an nearly flawless case" against him.
The scandal caused widespread anger among voters, leading to Najib's ouster during the May 9 general elections, effectively ending Barisan Nasional's 61-year rule in Malaysia.
Earlier, state news agency Bernama broadcast the movement of Najib's convoy live as it moved through rush-hour traffic during the drive from the Malaysian Anti-Corruption Commission's offices in the administrative capital, Putrajaya, to the court in Kuala Lumpur.
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After stumbling into the last-16 courtesy of a 2-1 win over Nigeria , the South American side were beaten 4-3 by France in Kazan.
Bridget Welsh, a Southeast Asia expert at John Cabot University in Rome, said Najib's arrest was the "inevitable outcome" after he lost power.
USA investigators say $4.5 billion was stolen and laundered from 1MDB by Najib's associates, including some $700 million that landed in Najib's bank account.
Najib came to power in 2009 and soon after founded 1MDB which is being investigated in at least six countries for alleged money laundering and graft.
According to an investigation by the Wall Street Journal, $10.6 million originating from SRC International Sdn Bhd, an energy company that was originally a subsidiary of 1MDB, was transferred to Najib's personal bank accounts, a fraction of hundreds of millions of dollars from 1MDB that was allegedly funnelled to him.
It is alleged that Riza used the money siphoned from 1MDB to fund his Oscar-nominated movie The Wolf of Wall Street starring Leonardo DiCaprio, among others.
Shortly after his downfall, a treasure trove of valuables was seized in raids on properties linked to Najib and his family - including cash, jewellery and luxury handbags - worth up to $273 million.