Hadith Qudsi 36
36) Narrated Abu Huraira The Prophet said: Once Paradise and Hell argued with one another. Hell said: I have been given preference by the oppressors and tyrants. Paradise said: What is the matter with me that only weak and humble people enter me. Allah said to the Paradise: You are My mercy. I will bestow mercy through you on whom I wish from among My slaves. And He said to Hell: Rather you are My torment. I will torment by you whom I wish from among My slaves. The Prophet added: Each of them will take its capacity. As for Hell, it will not be filled until Allah puts down His glorified foot in it. Then it will say: Enough! Enough! Enough! at that time all its parts will be filled up. Allah will not treat unjustly anyone of His creation. As regards Paradise, Allah will create other creations to fill it. (Being All-Merciful)
(This Hadfth is sound and reported by Bukhari and Muslim).
On Doomsday, Allah fills Hell by putting His Foot (a foot that suits his Glory and no human brain can perceive). As for Paradise he fills it by a new creation. This Hadith reveals that the mercy of Allah supercedes His anger, as He will not fill Paradise, too, by putting His Foot.