Is it obligatory to continue fulfilling a vow of two rak'ahs of prayer as a thank you after every prayer, following the Sunnah, for life, after recovering from an illness? And is it necessary to make up this prayer for the days of menstruation and post-natal bleeding? And is expiation obligatory for this vow?

Vows do not bring about anything that Allah has not decreed, and the Prophet, peace and blessings be upon him, forbade making vows. Furthermore, conditioning vows on the occurrence of a specific event is not from the Sunnah (definition). Nevertheless, whoever vows a vow of obedience must fulfill it, as the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him."

Therefore, the vow must be fulfilled unless there is an excuse such as illness or continuous incapacity, in which case an expiation for breaking an oath is due. There is no need to make up for days of menstruation and postnatal bleeding (nifas (definition)) in the vow because it is a legal excuse, and there is no prayer during those days.