The Malaysian Anti-Commission Commission (MACC) has stressed that it is not a paper tiger when it comes to investigating without fear or favour.

To illustrate this, MACC chief commissioner Abu Kassim Mohamed revealed that the commission even investigated the RM24.4 million ring, which the opposition claimed was purchased by Prime Minister Najib Abdul Razak's wife, Rosmah Mansor.

Abu Kassim said he personally instructed his officers to initiate a probe an hour after a report was lodged with MACC on the issue in 2011.

Two hours later, the MACC chief had called Najib and asked the latter if his wife had bought the ring.

The next day, MACC officers went to the Customs Department and it was confirmed that the ring had been sent back to New York.

"This is the first time we are openly revealing this," he said.

"I am telling you that we even probed the prime minister's wife, do you still think that we have anything to fear?" he was quoted as saying by Sin Chew Daily .

Abu Kassim was speaking at an event in Sabah yesterday, where both BN and opposition state assemblypersons inked an integrity pledge.

The opposition had alleged that Rosmah bought the import-duty-free multi-million ringgit diamond ring.

Rosmah denied the allegation while US-based jeweller Jacob & Co explained that the ring brought into the country under her name was not for her and she did not purchase it.

Jacob & Co said the 30.11-carat Natural Fancy Blue Grade Cushion Diamond Ring was for the viewing of Maira Nazarbayev, the mother-in-law of  Rosmah's daughter Nooryana Najwan.