Language:
كتاب التوحيد
97  
Oneness, Uniqueness of Allah (Tawheed)
(35)
Chapter: “…They want to change Allah’s Words….”
(35)
Sahih al-Bukhari 7503

Narrated Zaid bin Khalid:

It rained (because of the Prophet's invocation for rain) and the Prophet (ﷺ) said, "Allah said, 'Some of My slaves have become disbelievers in Me, and some others, believers in Me.'"

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا سُفْيَانُ، عَنْ صَالِحٍ، عَنْ عُبَيْدِ اللَّهِ، عَنْ زَيْدِ بْنِ خَالِدٍ، قَالَ مُطِرَ النَّبِيُّ صلى الله عليه وسلم فَقَالَ ‏ "‏ قَالَ اللَّهُ أَصْبَحَ مِنْ عِبَادِي كَافِرٌ بِي وَمُؤْمِنٌ بِي ‏"‏‏.‏
Reference : Sahih al-Bukhari 7503
In-book reference : Book 97, Hadith 128
USC-MSA web (English) reference : Vol. 9, Book 93, Hadith 594
  (deprecated numbering scheme)