What is the ruling on one who vowed not to watch pornographic films, then watched them, and what is the expiation for that?

Abandoning the viewing of pornography is a religious obligation. Vowing to abandon it reinforces this obligation. The majority of scholars hold that a vow to abandon something forbidden is not binding, and all that is required is repentance and seeking forgiveness. It is more prudent to offer an expiation for an oath, based on the Prophet's (peace be upon him) saying: "The expiation for a vow is the expiation for an oath." This involves feeding ten poor people, or clothing them, or freeing a slave. If one cannot find these, then fasting for three days.