Musnad Ahmad 30:3582

Uthman ibn Umar narrated to us, Abd al-Hamid ibn Ja'far narrated to us, from Yazid ibn Abi Habib, from Mu'awiyah ibn Mughith—or Mu'attib—from Abu Hurayrah, that he said: "O Messenger of Allah, what has your Lord, the Mighty and Majestic, returned to you regarding intercession?" He said: "I indeed thought that you would be the first to ask me about it, due to what I have seen of your eagerness for knowledge. My intercession is for whoever testifies that there is no god but Allah, sincerely, his heart confirming his tongue, and his tongue confirming his heart."