What is the criterion for ability (istita’ah) in fasting two consecutive months for the expiation of Dhihar (injurious assimilation of wife to a female relative) and sexual intercourse during a Ramadan day, which necessitates resorting to feeding [the poor], and is arduous, nearly continuous work considered an excuse?

"Ability" refers to the capacity to fast without unbearable hardship or fear of illness. Ordinary hardship does not nullify the obligation to fast. Permissible excuses for transitioning to feeding (fidya) include: old age, chronic illness, or severe hardship such as intense desire for intercourse, or fear of worsening an illness. Strenuous work permits transitioning from fasting to feeding, provided that fasting alongside it entails unbearable hardship or fear of harm to one's body, and that abstaining from work would cause financial detriment to one's livelihood. If these conditions are met and taking leave is not possible, then breaking the fast and feeding are permissible.