روى البخاري ومسلم أنه شكى إلى النبي صلى الله عليه وسلم الرجل يخيل إليه أنه يجد الشيء في الصلاة. قال: لا ينصرف حتى يسمع صوتاً أو يجد ريحاً
Imaam al-Bukhaari and Imaam Muslim each narrate in their Saheeh that a man complained to Rasoolullaah صلى الله عليه وسلم that, when he is performing Salaah, it appears to him as though he finds something (i.e. imagines that he has passed wind) in his Salaah." So Rasoolullaah صلى الله عليه وسلم said: "He should not turn away (i.e. not discontinue the Salaah) unless he hears a sound or finds a smell."
The meaning of this Hadeeth is that the person must be certain that he did, in fact, pass wind.
Explaining this Hadeeth, Imaam an-Nawawi رحمة الله عليه says:
"مَعْنَاهُ : يَعْلَم وُجُود أَحَدهمَا ، وَلَا يُشْتَرَط السَّمَاع وَالشَّمّ بِإِجْمَاعِ الْمُسْلِمِينَ ... وَلَا فَرْق فِي الشَّكّ بَيْن أَنْ يَسْتَوِي الِاحْتِمَالَانِ فِي وُقُوع الْحَدَث وَعَدَمه , أَوْ يَتَرَجَّح أَحَدهمَا , أَوْ يَغْلِب عَلَى ظَنّه , فَلَا وُضُوء عَلَيْهِ بِكُلِّ حَال" انتهى.
"What this means is that he knows that one of the two had taken place (i.e. he either heard something or smelt something). However, hearing or smelling has not been made a condition (for the Wudhoo to break) according to the consensus of the Muslims. There is no difference when it comes to the matter of Shakk (doubt), whether both possibilities of either the Wudhoo having broken or not is equal in his mind, or one of the two seems more possible, or his assumption is on one of them. There is no Wudhoo (necessary) upon him in any of these cases." [End quote.]
To summarise: For the person's Wudhoo to have broken, he must be certain that he did in fact pass wind and that it wasn't just his imagination. Sometimes, Shaytaan puts these Wasaawis (doubts and whispers) into the mind of a person to make things difficult for him and to make him doubt his Salaah, and doubt his Tahaarah, etc. He should ignore such doubts.
From the Usool al-Fiqh (Principles of Islaamic Jurisprudence) is the following:
اليقين لا يزول بالشك
"Conviction is not overruled by doubt."
If you are certain about something, then a simple doubt cannot remove it.
والله تعالى أعلم
والسلام