Principle of Completion
Principle of Completion (Arabic: قاعدة الفراغ) deals with religious acts that are composed of a number of different parts. If a person has finished the act and then doubts about whether or not they have performed that act, the whole act is assumed to be correct. This principle only applies if the person is in a state of forgetfulness or confusion. If the person doubts whether they intentionally left out a part of the act, then the principle is not considered to apply.
The reason for this is that most jurists consider this principle to be related to another principle, the Principle of Non-Forgetfulness, which is considered to be a rational principle that assumes a person didn't forget to do what they knew they were supposed to do.
A number of hadiths are offered in support of this principle, such as the sahih narration of Muhammad ibn Muslim from Imam Jafar al-Sadiq, when he was asked about a man who had finished praying and then doubted if he had done it correctly. The Imam responded: "He should not repeat it, and he has no obligation upon him". Another, more explicit narration, is the muthiqah narration of Imam Muhammad al-Baqir, where he says: "Whatever you doubt about after finishing your prayers, leave it, and do not repeat it". Another narration is the muthiqah narration of Muhammad ibn Muslim from Imam al-Baqir, where he says: "Any acts you doubt about which are now passed and finished, leave it as it is".