Musnad Ahmad 30:2621

Waki' narrated to us, he said: Ja'far ibn Burqan narrated to us, from Yazid ibn al-Asamm, from Abu Hurayrah, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, Allah, the Almighty and Majestic, says: 'I am as My servant expects of Me, and I am with him when he calls upon Me.'"