It was narrated that Ibn ‘Abbas (رضي الله عنه) said:

Two men disputed and one of them had to swear an oath, so he swore by Allah besides Whom there is no other God that (his opposite number) had no right over him. Then Jibreel came down and said: Tell him to give him his due, for the other one is in the right and he is lying. And the expiation for his oath is his acknowledgement or testimony that there is no God but Allah.