Musnad Abu Ya'la al-Mawsili 112:31

Harun ibn Abdullah narrated to us, Hammad ibn Mas'adah narrated to us, from Imran al-Qummi, from al-Hasan, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "I will continue to intercede with my Lord, and He will grant me intercession, until I say: 'O Lord, grant me intercession for whoever said: There is no god but Allah.' He said: Then He will say: 'This is not for you, O Muhammad. It is only for Me. By My Might, My Forbearance, and My Mercy, I will not leave in the Fire anyone—or he said: any servant—who said: There is no god but Allah.'"