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Narrated Abu Huraira: I heard Allah's Messenger (ﷺ) saying, "Before Allah created the creations, He wrote a Book (wherein He has written): My Mercy has preceded my Anger." and that (Book) is written with Him over the Throne."
Source: sunnah.com
Other translations
Arabic
وَقَالَ لِي خَلِيفَةُ بْنُ خَيَّاطٍ حَدَّثَنَا مُعْتَمِرٌ، سَمِعْتُ أَبِي، عَنْ قَتَادَةَ، عَنْ أَبِي رَافِعٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لَمَّا قَضَى اللَّهُ الْخَلْقَ كَتَبَ كِتَابًا عِنْدَهُ غَلَبَتْ ـ أَوْ قَالَ سَبَقَتْ ـ رَحْمَتِي غَضَبِي. فَهْوَ عِنْدَهُ فَوْقَ الْعَرْشِ ".
sunnah.com