What is the difference between a person afflicted with obsessive-compulsive disorder (waswasi) basing his actions on the most frequent thought, and basing them on the first thought that comes to mind? And should a person with waswas base his actions on the first thought even if his thoughts become numerous during prayer, ablution, or other acts of worship?

The intention of the one who utters the aforementioned phrase is to act upon the first thought, whether it is the lesser or the greater. The Hanbalis hold that a person afflicted by waswas (obsessive whispering/doubts) should disregard doubt in all acts of worship and pay no attention to it. So, if doubt becomes frequent to the point of becoming waswas, it should be dismissed and ignored. This applies to prayer, ablution (wudu'), ritual bathing (ghusl), and removing impurities. This is the view mentioned by Ibn Abi Musa and those who followed him, and it is what should be acted upon to close the door to waswas.