What is the ruling on one who prayed an obligatory prayer while menstruating out of forgetfulness, and is there any fidyah or expiation due upon her?

Forgetting is not something for which Allah holds one accountable. Allah the Almighty said: "Our Lord, do not hold us accountable if we forget or make a mistake." And in response to this, He said: "I have done so." As the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my nation for mistakes, forgetfulness, and what they are forced to do." Based on this, if a menstruating woman prays out of forgetfulness, there is no sin upon her, nor any expiation, whether it be for the Dawn prayer or any other prayer.