Kindly refer to the following link(s): 


Things that invalidate wudoo’ (IslamQ&A)


What are the things that nullify my ablution (wudu)?


Praise be to Allaah.  

What invalidates wudoo’ is wind that comes out of the back passage. This is what invalidates wudoo’, because according to the hadeeth of ‘Abd-Allaah ibn Zayd, a man thought that he had broken his wudoo’ whilst praying when he had not done so, and the Prophet (peace and blessings of Allaah be upon him) said: “Do not stop praying until you hear a sound or smell an odour.” If something happens in his stomach and he is not sure whether he has broken his wudoo’ or not, then the basic principle is that he is still pure (still has wudoo’).





And Allah Knows Best