What is the solution for the problem of obsessive-compulsive disorder (Waswas al-Qahri) regarding intention during ablution (Wudu), ritual bath (Ghusl), and prayer (Salat)? What is intention (Niyyah)? Is it a pillar (Rukn) in acts of worship? Does changing or invalidating the intention during an act of worship affect its validity?

Intention (Niyyah) is a pillar in acts of worship, and its place is the heart. It is merely a person's intent for an action and their knowledge of it. So, whoever knows what they intend to do has already made the intention. Every action performed by a sane person must have an intention. Invalidating the intention nullifies the act of worship if the intention to discontinue is firm, such as someone who intends to stop praying or to break their fast. Changing the intention may invalidate the act in some cases. For example, if someone intended to pray Asr and then changed it to Dhuhr, it would be invalidated. However, if someone prayed a fard (obligatory) prayer and intended it as an absolute nafl (supererogatory) prayer during the prayer, it would be valid as a nafl prayer.