It was narrated that Abu Sa'eed Al-Khudri said:
"The Prophet was asked about doubts (concerning ablution) during prayer. He said: 'he should not leave until he hears a sound or detects an odor.'" (FR)
English reference (FR): Vol. 1, Book 1, Hadith 514 (FR)
Arabic reference (FR): Book 1, Hadith 554 (FR)