What is the meaning of the jurists' saying: "Doubt does not necessitate obligation," and what are examples of that?

The jurists' statement that doubt does not necessitate obligation means that the presumed original state (al-asl al-mustashab) does not change. An example of this is the command to wash the hands three times upon waking from sleep, which is for recommendation (nadb), not obligation (wujub), because the original state (al-asl) is the purity of the hand. The hadith justified the washing by doubt, so one does not shift from the original purity of the hand to the obligation of washing it merely due to doubt about its impurity.