Musnad Abu Ya'la al-Mawsili 65:11

Salih ibn Hatim ibn Wardan narrated to us, Mu'tamir ibn Sulayman narrated to us, he said: I heard my father narrating from Abu 'Imran al-Jawni, from Jundub ibn 'Abd Allah al-Bajali, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "A man said: 'By Allah, Allah will not forgive so-and-so.' Then Allah said: 'Who is this who swears by Me that I will not forgive so-and-so? For I have forgiven so-and-so, and I have nullified your deed.'"