In english
On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Messenger of Allah (peace and blessings of Allah be upon him) said:
"When Allah decreed the Creation He pledged Himself by writing in His book which is laid down with Him: 'My mercy prevails over my wrath.'"
In Arabic
عَنْ أَبِي هُرَيْرَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ:
" لَمَّا قَضَى اللَّهُ الْخَلْقَ، كَتَبَ فِي كِتَابِهِ عَلَى نَفْسِهِ، فَهُوَ مَوْضُوعٌ عِنْدَهُ: إِنَّ رَحْمَتِي تَغْلِبُ غَضَبِي"
رواه مسلم (وكذلك البخاري والنسائي وابن ماجه)