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