Hazare's comments came after Singh announced that he would quit public life if any of the allegations raised by Team Anna against him is proved.
"If the Prime Minister is an honest man, then he should be ready to face probe. I always saw the Prime Minister as a non-corrupt personality," the Gandhian told a TV channel.
He said he needed more time to study allegations levelled against the Prime Minister by his team.
The inquiry into the charges against the Prime Minister should be headed by a former judge, he said.
Team Anna had on last Saturday demanded an investigation by a Special Investigation Team of three retired judges into allegations of corruption raised by them against Singh and 14 of his colleagues in the Union Cabinet.