What is the ruling on one who doubts whether he swore an oath not to do a certain thing, even though he did not swear an oath in the first place?

Whoever doubts whether he swore an oath or not, the default is that no oath was taken and nothing is incumbent upon him. However, it is advisable to consult scholars to hear what happened, as doubt about an oath was mentioned, followed by swearing to others to prove that that thing was never done.