Praise be to Allah.
Yes, the scholars are unanimously agreed that fainting is one of the things that invalidate wudoo’, even if it is only brief.
Whoever faints and loses consciousness, even for a moment, invalidates his wudoo’.
Ibn Qudaamah said in al-Mughni (1/234):
Loss of consciousness due to insanity, fainting, intoxication and medicines that cause loss of consciousness invalidates wudoo’ whether the duration is brief or long, according to scholarly consensus. Ibn al-Mundhir said: the scholars are unanimously agreed that one who has fainted must do wudoo’.
The loss of consciousness in these cases is greater than that of one who goes to sleep, because they do not respond to any stimuli. The fact that wudoo’ is required of one who sleeps indicates that it is also required in cases that are more severe. End quote.
Al-Nawawi said in al-Majmoo’ (2/25):
The ummah is unanimously agreed that wudoo’ is invalidated by insanity and by fainting. This consensus was narrated by Ibn al-Mundhir and others. Our companions are agreed that if a person loses consciousness due to insanity, fainting, sickness, intoxication caused by wine or nabeedh, etc, or drinking medicine due to necessity etc, then his wudoo’ is invalidated.
Our companions said: the kind of intoxication that invalidates wudoo’ is that in which a person loses his senses, not the initial feelings of pleasure. And our companions said: and it makes no difference whether a person is sitting on his backside or is in any other position, or is makes not difference whether the loss of consciousness is brief or long. End quote.
Shaykh Ibn ‘Uthaymeen was asked: Is wudoo’ invalidated by fainting?
He replied: Yes, wudoo’ is invalidated by fainting, because fainting is more severe than sleeping, and sleeping invalidates wudoo’ if it is so deep that the sleeper does not know whether anything has come out of him or not. As for light sleep in which the sleeper feels it if anything comes out of him, that does not invalidate wudoo’, whether he is lying down, sitting and reclining, or sitting without reclining, or in any other position. So long as he can feel it if he breaks his wudoo’, his sleep does not invalidate his wudoo’. But fainting is more severe than sleep, so if a person faints then he has to do wudoo’. End quote.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 11/200
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: What is the ruling on those who go into a daze for a few moments?
That depends. If it is something brief where a person does not lose consciousness and it does not prevent him feeling it if he breaks his wudoo’, then it does not matter, as in the case of one who becomes drowsy but does not sleep deeply, rather he can hear sounds around him. In this case it does not matter, as he will know if something comes out of him. The same applies if the daze does not prevent him feeling things. But if the daze means that he will not feel if anything comes out of him, as in the case of one who is intoxicated or one who is afflicted with a sickness that causes him to lose his senses so that he is in a state of unconsciousness, then this invalidates his wudoo’ as in the case of fainting or those who suffer epileptic seizures. End quote.
Fataawa al-Shaykh Ibn Baaz, 10/145.