What is the legal evidence upon which jurists based their ruling that one who frequently doubts during acts of worship should base their actions on the greater number, not the lesser?

Excessive doubt in acts of worship comes from the whispers of Satan. The remedy for it is to ignore it and not pay attention to it, for Satan intends to corrupt a Muslim's religion and mind with these whispers. The Prophet, peace and blessings be upon him, guided those afflicted by this to seek refuge with Allah and to stop these thoughts. Whispering is a pervasive disease whose effective cure is to completely ignore it and to stop listening to it, because it increases with attention until it drives its owner into the realm of the insane. The person who suffers from excessive doubt is one who doubts frequently, and it is obligatory for him to ignore doubt and not pay attention to it. If he doubts in his prayer, he should build upon the greater number; for example, if he doubts whether he prayed three or four rak'ahs, he should consider it four. This is the more preponderant opinion among scholars regarding the person who suffers from excessive doubt. Furthermore, the most beneficial cure for repelling whispering is to turn to the remembrance of Allah Almighty and to do it frequently.