Musannaf Ibn Abi Shayba 7:175

Hafs narrated to us, from al-Hajjaj, from al-Hakam, from Ibrahim, regarding a man who swears an oath about a thing that is with him, while he does not know that it is with him: He said, "He must offer expiation for his oath." He said: And 'Ata' and al-Hakam said regarding the one who does not offer expiation: "He has committed an act of disbelief."