The Duty of a Muslim after Apostasy
Issue No. 1427- If an insane person recovers and becomes sane, it will not be obligatory to offer qaḍā for the fasts which he did not observe when he was insane. Similarly, if a non-Muslim embraces Islam, it is not obligatory to offer qaḍā for the fasts of the period during which he was not a Muslim. However, if a Muslim apostatises (becomes Murtad) and becomes Muslim again, he must perform qaḍā for the fasts of the period during which he was an apostate.
