Former prime minister Manmohan Singh is stable and under observation at the AIIMS here after suffering reaction to a new medication and developing fever, hospital sources said on Monday. The 87-year-old Congress leader was admitted to the hospital on Sunday evening after he complained of uneasiness. The sources said that Singh had developed a reaction to a new medication and further investigation is being carried on him to rule out other causes of fever. "Dr Manmohan Singh was admitted for observation and investigation after he developed a febrile reaction to a new medication," the sources said. "He is being investigated to rule out other causes of fever and is being provided care as needed. He is stable and under care of a team of doctors at the Cardiothoracic Centre of AIIMS," they said. Read in Malayalam here "All his parameters are fine. He is under observation at the AIIMS," a source close to him has said. Singh, a senior leader of the opposition Congress, is currently a Member of Rajya Sabha from Rajasthan. He was the prime minister between 2004 and 2014. In 2009, Singh underwent a successful coronary bypass surgery at the AIIMS. A number of leaders expressed have expressed concern over his health and wished him a speedy recovery.