Musnad Ahmed: Hadith 2678

Two men referred a dispute to the Messenger of Allah (ﷺ) and one of them had to swear an oath, so he swore by Allah besides Whom there is no other god that he did not owe anything (to the other man). Then Jibreel came to the Prophet (ﷺ) and said: He is lying; he does owe him something. So he told him to give him his dues, and the expiation for breaking his oath was his affirming that there was no god except Allah, or his shahadah.