If a person sleeps and is not sure whether this sleep invalidates wudoo’ or not, then wudoo’ is not invalidated by that.

Al-Nawawi (may Allaah have mercy on him) said in al-Majmoo’ (2/17):

If a person is not sure whether he fell asleep or dozed off, but one of the two happened, his wudoo’ is not invalidated. Al-Shaafa’i said in al-Umm: To be on the safe side, he should do wudoo’… then he said:

If he is certain that he slept, but he is not sure whether he was sitting firmly on his backside or not, he does not have to do wudoo’. This was clearly stated by the author of al-Bayaan and others, and it is the correct view. End quote. 

The words of al-Nawawi (may Allaah have mercy on him) – “whether it he was sitting firmly on his backside or not” – mean: whether he was sitting firmly on his backside on the ground, on the basis that he will not invalidate his wudoo’ if he is sitting firmly on his backside on the ground. In the answer to question no 36889 , we have stated that the correct view concerning the invalidation of wudoo’ by sleep is that if the sleep is deep, it invalidates wudoo’, and if it is light it does not do so. 

The evidence that sleep does not invalidate wudoo’ in the case of uncertainty as to whether anything that invalidates it has happened is the report narrated by al-Bukhaari (137) and Muslim (361) from ‘Abbaad ibn Tameem from his paternal uncle, who said: A complaint was made to the Prophet (S) about when one thinks that something has happened whilst he praying. He said: “Do not stop until you hear a sound or notice a smell. 

And Muslim (362) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (S) said: “If one of you feels something in his stomach and is not sure whether something came out of him or not, let him not leave the mosque unless he hears a sound or notices a smell.” 

Al-Nawawi (may Allaah have mercy on him) said in Sharh Muslim: 

The words of the Prophet (peace and blessings of Allaah be upon him): “unless he hears a sound or notices a smell” mean: unless he is certain that one of them has happened. It is not essential that he both hear and smell it, according to the consensus of the Muslims. 

This hadeeth is one of the basic principles of Islam and one of the most important principles of fiqh, which is that things are assumed to remain as they originally were, unless there is proof to the contrary, and mere doubt does not change anything. 

An example of that is the issue which is mentioned in the hadeeth: which is that a person who is certain that he has purified himself and is not sure whether that purity had been broken is ruled to still be in a state of purity, and it makes no difference whether this doubt occurs during the prayer or outside of prayer. This is our view and the view of the majority of scholars of the earlier and later generations. 

Our companions said: It makes no difference whether the probability of the wudoo’ being broken and the probability of it not being broken are equal, or one of them is more likely, or he thinks that one of them is more likely. He does not have to do wudoo’ in any case.  

And our companions said: But it is mustahabb for him to do wudoo’, in order to be on the safe side. End quote.