This is how I heard. At one time, the Blessed One was living in the city of Sāvatthi, in Jeta’s Park, at Anāthapiṇḍika’s monastery. A number of monks approached the Blessed One, worshipped Him respectfully and sat down to one side. They asked the Blessed One, “Bhante, how many eons have passed by and gone by?”
“Monks many eons have passed by and gone by. It is not easy to count them and say there are so many eons, so many hundreds of eons, so many thousands of eons, or so many hundreds of thousands of eons.”
“Then, Bhante, is it possible to give a simile to know how many eons have passed by and gone by?”
“It is possible, monks,” The Blessed One said. “Suppose, monks, there were four disciples near me each with a life span of a hundred years, living a hundred years and each day they were each to recollect a hundred thousand eons. There would still be eons not yet recollected by them. But those four disciples, each with a life span of a hundred years, living a hundred years, would pass away at the end of a hundred years. It is not easy to count them and say that there are so many eons, or so many hundreds of eons, or so many thousands of eons, or so many hundreds of thousands of eons.
“What is the reason for that? It is because, monks, this cycle of rebirth is endless. The beginning of this extremely long journey cannot be discovered. These beings, hindered by lack of knowledge of the true nature of life and bound by craving, roam and wander on in this endless journey.
“For such a long time, monks, you have experienced various types of suffering, tragedies, and disasters. You have filled the cemetery with your dead bodies. Therefore, monks, the time has come for you to understand the meaningless nature of all conditioned things. The time has come for you to become detached from them. And the time has come for you to be liberated from them.”