If you fart or hear, smell someone's fart during your namaaz

Volume 1, Book 4, Number 137:

Narrated Abu Huraira:

Allah's Apostle said, "The prayer of a person who does ,Hadath (passes, urine, stool or wind) is not accepted till he performs (repeats) the ablution." A person from Hadaramout asked Abu Huraira, "What is 'Hadath'?" Abu Huraira replied, " 'Hadath' means the passing of wind from the anus."

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Volume 1, Book 4, Number 138:
Narrated Nu'am Al-Mujmir:

Once I went up the roof of the mosque, along with Abu Huraira. He perform ablution and said, "I heard the Prophet saying, "On the Day of Resurrection, my followers will be called "Al-Ghurr-ul-Muhajjalun" from the trace of ablution and whoever can increase the area of his radiance should do so (i.e. by performing ablution regularly).' "

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Volume 1, Book 4, Number 139:
Narrated 'Abbas bin Tamim:

My uncle asked Allah's Apostle about a person who imagined to have passed wind during the prayer. Allah' Apostle replied: "He should not leave his prayers unless he hears sound or smells something."

-from Hadiths of Sahih Bukhari http://www.usc.edu/dept/MSA/fundamentals/hadithsunnah/bukhari/004.sbt.html