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