كتاب الذكر والدعاء والتوبة والاستغفار
48  
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
(6)
Chapter: The Virtue Of Remembrance, Supplication, Drawing Close To Allah And Thinking Positively Of Him
(6)
باب فَضْلِ الذِّكْرِ وَالدُّعَاءِ وَالتَّقَرُّبِ إِلَى اللَّهِ تَعَالَى
Sahih Muslim 2675 d

Abu Huraira reported Allah's Messenger (may peace be upon him as saying that Allah thus stated:

I live in the thought of My servant as he thinks of Me and with him as he calls Me.
حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا وَكِيعٌ، عَنْ جَعْفَرِ بْنِ بُرْقَانَ، عَنْ يَزِيدَ، بْنِ الأَصَمِّ عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِنَّ اللَّهَ يَقُولُ أَنَا عِنْدَ ظَنِّ عَبْدِي بِي وَأَنَا مَعَهُ إِذَا دَعَانِي ‏"‏ ‏.‏
Reference : Sahih Muslim 2675d
In-book reference : Book 48, Hadith 25
USC-MSA web (English) reference : Book 35, Hadith 6495
  (deprecated numbering scheme)