Leave what makes you in doubt for what does not make you in doubt

Al-Hasan bin 'Ali, may Allah be pleased with them, said:

This is an example of the all-inclusive Hadiths of the Prophet, peace and blessings of Allah be upon him, it combines many meanings that are clear for he who contemplates, despite the fact that it is a brief summary it is considered the base of the rules which determine what is permissible and what is prohibited, concerning the chapters of purity, prayer, fasting, Zakah, pilgrimage, and the other acts of worship and transactions, the Jurist can not do without this very rule since it is the focus of the eyes of the Jurists and those who seek fatwas when the doubt conflicts with certainty.

Among the rules that fall under this Hadith, the saying of the Jurists: 
Certainty is not waved out by doubt. 

Certainty is waved out only by another certainty..
Taking along the original ruling, casting doubt and resuming the previous ruling
In principle: if one doubts whether or not he did something the ruling is that he did not do it.
The wrong assumptions does not count.