Malaysia's Prime Minister Najib Razak and then Deputy Prime Minister Muhyiddin Yassin at UMNO party headquarters in Kuala Lumpur on May 6, 2013, after winning that year's elections. Reuters

Malaysia's former deputy prime minister, who was fired after he raised doubts about an investigation linked to Prime Minister Najib Razak, said Thursday he would stay in the ruling party but fight his suspension as its deputy president. There was speculation he would quit the United Malays National Organization (UMNO), following longtime leader Mahathir Mohamad.

"I will stay in UMNO, it's in my blood," Muhyiddin Yassin said at a press conference, Channel News Asia reported.

"I'll stay on to fight the party's evil from within," the Straits Times quoted Muhyiddin as saying. "Just because I plan to stay on in UMNO doesn't mean I won't do anything. Our fight is not over."

On Wednesday, Muhyiddin's successor as deputy prime minister called on him not to make hasty decisions.

Muhyiddin was Najib's heir apparent until July last year, when he said investigations into a debt-ridden state-run fund were being delayed. The fund was later linked to $681 million that had been deposited in Najib's personal account. Muhyiddin lost the deputy prime minister post then, and the deputy party president post last week, after saying he had seen criminal evidence related to Najib's account.

In January, the current attorney general — the previous one was fired from the cabinet with Muhyiddin last year — cleared Najib, saying he confirmed the prime minister's claim that the money came from unidentified members of Saudi Arabia's royal family.

The Wall Street Journal, which first reported on the $681 million, said this week deposits into Najib's accounts actually exceeded $1 billion and came mostly from 1Malaysia Development Berhad (1MDB), the infrastructure fund created by Najib, and whose advisory board he still heads.

On Wednesday, the Asia Sentinel detailed examples of how money flowed to politicians in Malaysia, including 50,000 Malaysian ringgit ($12,000) a month each to 191 local district chiefs, totaling $27 million, from the prime minister's personal accounts. The Sentinel said more money — mostly from the 1MDB foundation — flows through the Village Security and Development Committee and four agencies under the Ministry of Information, Communications and Culture.

The Sentinel said Najib didn't start the political money machine but he expanded a system in place during the time of his predecessors including Mahathir Mohamad. Mahathir, who has been calling for Najib to step down since last year, resigned from UMNO this month.

Muhyiddin isn't the first heir apparent to lose his place in Malaysia. In 1998, Deputy Prime Minister Anwar Ibrahim was ousted, convicted and jailed on corruption and sodomy allegations. The prime minister who ousted Anwar was Mahathir.