Musnad Ahmad 27:912

'Affan narrated to us, Hammad—meaning Ibn Salamah—narrated to us, he said: Thabit informed us, from 'Abdullah ibn 'Umar, that the Messenger of Allah (ﷺ) said to a man: "Did you do such and such?" He said: "No, by the One 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 he has been forgiven by his saying: 'There is no god but Allah' (La ilaha illa Allah)." Hammad said: [Thabit] did not hear this from Ibn 'Umar; there is a man between them—meaning Thabit.