Hadith 15, 40 Hadith Qudsi
On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Prophet (ﷺ) said:
Allah the Almighty said:
I am as My servant thinks I am (1). I am with him when he makes mention of Me. If he makes mention of Me to himself, I make mention of him to Myself; and if he makes mention of Me in an assembly, I make mention of him in an assembly better than it. And if he draws near to Me an arm's length, I draw near to him a cubit, and if he draws near to Me a cubit, I draw near to him a fathom. And if he comes to Me walking, I go to him at speed.
(1) Another possible rendering of the Arabic is: "I am as My servant expects Me to be". The meaning is that forgiveness and acceptance of repentance by the Almighty is subject to His servant truly believing that He is forgiving and merciful. However, not to accompany such belief with right action would be to mock the Almighty.

It was related by al-Buhkari (also by Muslim, at-Tirmidhi and Ibn-Majah).
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : 

"يَقُولُ اللَّهُ تَعَالَى: أَنَا عِنْدَ ظَنِّ عَبْدِي بِي، وَأَنَا مَعَهُ إِذَا ذَكَرَنِي، فَإِنْ ذَكَرَنِي فِي نَفْسِهِ، ذَكَرْتُهُ فِي نَفْسِي، وَإِنْ ذَكَرَنِي فِي مَلَإٍ، ذَكَرْتُهُ فِي مَلَإٍ خَيْرٍ مِنْهُمْ، وَإِنْ تَقَرَّبَ إِلَيَّ بِشِبْرٍ، تَقَرَّبْتُ إِلَيْهِ ذِرَاعًا، وَإِنْ تَقَرَّبَ إِلَيَّ ذِرَاعًا، تَقَرَّبْتُ إِلَيْهِ بَاعًا(1) وَإِنْ أَتَانِي يَمْشِي، أَتَيْتُهُ هَرْوَلَةً" 
رواه البخاري (وكذلك مسلم والترمذي وابن ماجه)