Musnad Ahmad 30:1737

Sulayman narrated to us, Isma'il narrated to us, 'Amr narrated to me, from Sa'id, from Abu Hurayrah, who said: I said to the Prophet, peace and blessings of Allah be upon him: "Who is the happiest of people with your intercession on the Day of Resurrection?" The Prophet, peace and blessings of Allah be upon him, said: "I indeed thought, O Abu Hurayrah, that no one would ask me about this hadith before you, due to what I have seen of your eagerness for the hadith. The happiest of people with my intercession on the Day of Resurrection is the one who says: 'There is no god but Allah,' sincerely from his own soul."