Musnad Ahmad 32:293

Yazid narrated to us, Shu'bah informed us, from Qatadah, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Your Lord said: 'If My servant draws near to Me by a span, I draw near to him by a cubit; and if he draws near to Me by a cubit, I draw near to him by an arm's length; and if he comes to Me walking, I come to him at a run.'"