Sahih Muslim
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Book Number: 035 / Hadith Number: 6498
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying that Allah, the Exalted and Glorious, thus stated: I live in 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 the assembly, better than he (does that), 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 it he walks towards Me, I rush towards him.