Musnad Abu Ya'la al-Mawsili 154:60

Abu Khaythama narrated to us, Affan narrated to us, Hammad narrated to us, Thabit narrated 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?" The man said, "No, by the One besides Whom there is no god, I did not do it." Then Gabriel said to him, "He has indeed done it, but Allah, Blessed and Exalted is He, has forgiven him for his saying, 'There is no god but Allah.'" Hammad said: This was not heard directly from Ibn Umar; there is a man between them.