What is the ruling on one who forgot that he swore that if he did something, he would fast for a month?

Scholars differ concerning one who breaks an oath forgetfully, whether expiation is incumbent upon him or not. The correct view is that it is not incumbent upon him, due to the generality of the Almighty's saying: "Our Lord, do not impose blame upon us if we forget or make a mistake." And Allah said in response to it: "I have done so." However, if he does what he swore to abandon while remembering his oath, then one of two matters is necessary for him: either fasting for a month, or offering expiation for an oath.