Musnad Abu Ya'la al-Mawsili 116:18

Ahmad narrated to us, Abu Dawud narrated to us, Shu'bah narrated to us, from Qatadah, who heard Anas narrating that the Messenger of Allah (may Allah bless him and grant him peace) said: Allah the Exalted said: "I am as My servant expects of Me, and I am with him when he calls upon Me."