Is it obligatory to fast for two months for someone who made a vow to do so to avoid a sin, then fell into that sin despite the difficulty?

A vow made in the heat of an argument or out of anger, often called a vow of obstinacy or rage, does not necessitate fulfillment. One is given the choice between fulfilling it or offering an expiation for breaking an oath. This is the view of the Shafi'i school, the well-known position of the Hanbali school, and one of the two narrations from Abu Hanifa, which is the preferred opinion. It is imperative to guard against sins and to offer sincere repentance.