It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "If the people of Hell were told: 'You will remain in Hell for (a period equal to) the number of every pebble in the world,' they would rejoice at it; and if the people of Paradise were told: 'You will remain in Paradise for (a period equal to) the number of every pebble in the world,' they would grieve at it. But (the truth is that) eternity has been decreed for them."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10346