What is the ruling on a husband saying that it is permissible to combine prayers due to extreme fatigue, and is this considered an act of deeming the forbidden permissible (istihlal) and a cause for disbelief (kufr), especially since the previous fatwa stated that whoever utters disbelief jestingly is a disbeliever, but did not definitively rule on the occurrence of disbelief in another case?

Some jurists have permitted combining two prayers due to extreme fatigue, but it is not permissible to combine all prayers. Whoever deems lawful what is forbidden by consensus commits disbelief (kufr), otherwise, he might be interpreting the concession due to an excuse. The questioner is advised to abandon researching issues of disbelief (takfir) because of obsessive thoughts (waswas); for erring on the side of forgiveness is easier than erring on the side of takfir.