"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.
"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 have expressed concern over his health and wished him a speedy recovery.