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