Musnad Ahmad 30:3830

Kathir ibn Hisham narrated to us, Ja'far ibn Burqan narrated to us, Yazid ibn al-Asamm narrated to us, from Abu Hurayrah, who attributed it to the Prophet, may Allah bless him and grant him peace, who said: "Allah, the Mighty and Majestic, says: 'My servant is as he thinks of Me, and I am with him when he calls upon Me.'"