Musnad Ahmad 27:1642

Affan narrated to us, Hammad ibn Salamah narrated to us, Thabit narrated to us, from Abdullah ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said to a man: "Did you do such and such?" He said: "No, by Allah, besides whom there is no god, I did not do it." He said: Then Gabriel (peace be upon him) said to him: "He did do it, but Allah the Exalted has forgiven him because of his saying 'There is no god but Allah'." Hammad said: "He did not hear this from Ibn Umar; there is a man between them," meaning Thabit.