Musnad Ahmad 30:951

Hashim and al-Khuza’i—meaning Abu Salamah—said: Layth narrated to us, Yazid ibn Abi Habib narrated to me, from Salim ibn Abi Salim, from Mu’awiyah ibn Mu’attib al-Hudhali, from Abu Hurayrah: that he heard him say: I asked the Messenger of Allah, peace and blessings of Allah be upon him: "What has your Lord returned to you regarding intercession?" He said: "By the One in Whose hand is the soul of Muhammad, I truly thought that you would be the first of my nation to ask me about that, due to what I have seen of your eagerness for knowledge. By the One in Whose hand is the soul of Muhammad, their crowding at the gates of Paradise does not concern me more than the completion of my intercession. And my intercession is for whoever testified that there is no god but Allah, sincerely, his heart confirming his tongue, and his tongue confirming his heart."