On the authority of Abu Harayrah (may Allah be pleased with him), who said that the Prophet (PBUH) 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 assemble better than it. And if he draws near to Me an arm's length, I draw near to him a fathom's length. 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).