Sahih Muslim Book 35, Hadith Number 6471.
Chapter : Exhortation for the remembrance of Allah, the Exalted.
Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying that Allah, the Exalted and Glorious, thus stated: I am near to the thought of My servant as he thinks about Me, and I am with him as he remembers Me. And if he remembers Me in his heart, I also remember him in My Heart, and if he remembers Me in assembly I remember him in assembly, better than his (remembrance), and if he draws near Me by the span of a palm, I draw near him by the cubit, and if he draws near Me by the cubit I draw near him by the space (covered by) two hands. And if he walks towards Me, I rush towards him.