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.'" (RU)
English reference (RU): Vol. 1, Book 1, Hadith 514 (RU)
Arabic reference (RU): Book 1, Hadith 554 (RU)