Angry that a fellow prison inmate had got him in trouble, Mustafa Ya'acob, 58, emptied a pail of hot water on the man.
The victim was rushed to Changi General Hospital with burns on 17 per cent of his total body surface area, including his genitals and thighs, and had to undergo skin grafting.
Yesterday, Mustafa started serving a 30-month sentence for the 2012 incident at Changi Prison.
He pleaded guilty to one count each of voluntarily causing grievous hurt, failure to remain indoors during restricted hours, causing mischief and leaving Singapore without written authority from the Criminal Investigation Department (CID) director.
Mustafa had spotted his 51-year-old victim, a gang member, assaulting a prison officer on Jan 23, 2012.
But he did nothing to stop the assault as he did not want to cause any trouble with his "brothers".
Mustafa, who was serving a sentence for drug-related offences, also belonged to the same secret society.
His failure to intervene - the prison department requires inmates to do so - led to Mustafa being reprimanded two days later. He was told he would be sent to an isolated segregated cell as a punishment.
After leaving the reprimanding officer's office, he went into the prison's mess area, where he saw the victim.
Angry at the turn of events, Mustafa went to the kitchen area, filled a pail with hot water and poured it on him.
A scuffle broke out and prison wardens had to restrain Mustafa.
The victim was rushed to Changi General Hospital and had to be warded twice before he was discharged almost a month later.
A Singapore Prison Service (SPS) spokesman told The New Paper incidents of assault between inmates do occur in prison, although they are not common.
The spokesman said: "Our officers are professionally trained to quell such situations to minimise injury to inmates and officers.
SPS does not tolerate any form of violence, be it against staff or other inmates."
To prevent a recurrence of the incident, SPS has reviewed its operational processes and procedures.
Mustafa, who had served his drug sentence from Dec 9, 2002, was released from prison on Dec 2 last year to undergo three years' police supervision.
He was electronically tagged on Jan 16 this year and was supposed to remain indoors at his Toa Payoh flat between 7pm and 6am every day. He was also barred from leaving the country without written authority from the CID director.
He was supposed to plead guilty in court on April 2 for the scalding offence but failed to show up. As a result, a warrant was issued for his arrest. He had cut off the rubber strip and the security clip of the tagging device around his ankle on April 1 and left his flat.
With the help of some associates, he boarded a sampan at Jurong Port on April 6 and illegally made his way to Batam, Indonesia.
After that, he went to Johor but was later arrested by the Malaysian authorities on April 30.
He was sent back to Singapore soon afterwards to face the music.
For voluntarily causing grievous hurt, Mustafa could have been jailed for life or jailed up to 15 years and fined. He will not be caned as he is above 50 years old.
This article was first published on July 1, 2014.
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