A tradition recorded in the books of Hadith is set forth here in the words of Musnad Ahmad: “On the Day of Judgement, when all the people of Paradise have entered Paradise and all the people of Hell have entered Hell, Death will be brought there in the form of a white sheep. It will be made to stand between Heaven and Hell. Then it will be said, ‘O People of Paradise, do you recognize it?’ They will look up to it and say, ‘Yes, it is Death.’ After this the people of Hell will be asked, ‘O People of Hell, do you recognize this?’ They will look up and say, ‘Yes, it is Death.’ After this the order will be given for Death to be slaughtered. Then, it will be said, ‘O People of Paradise, now you have eternity and no death, and O People of Hell, now you have eternity and no death.’” (Musnad Ahmad, Hadith No. 11066)
What is purification? Purification means making oneself a purified soul, such as will be worthy of inhabiting the refined environment of Paradise. On Doomsday, the purified individuals will be ushered into Paradise and the unpurified individuals will be cast into Hell. After this it will be announced that the law of death has been ended. Now both parties have to abide eternally in their places. This will be a unique moment. The people of Paradise will be in a state of bliss for having found the eternal world of happiness. On the contrary, the people of Hell will experience indescribable remorse—an eternal torment over what has become their lot because of not having purified their souls. This perception is undoubtedly a powerful incentive to undertaking the process of purification.
Source: Discovering God