The Book of Purification and its Sunnah
1/74. Chapter:
No ablution except for impurity
Sunan Ibn Majah
حَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا الْمُحَارِبِيُّ، عَنْ مَعْمَرِ بْنِ رَاشِدٍ، عَنِ الزُّهْرِيِّ، أَنْبَأَنَا سَعِيدُ بْنُ الْمُسَيَّبِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ سُئِلَ النَّبِيُّ ـ صلى الله عليه وسلم ـ عَنِ التَّشَبُّهِ فِي الصَّلاَةِ فَقَالَ " لاَ يَنْصَرِفْ حَتَّى يَسْمَعَ صَوْتًا أَوْ يَجِدَ رِيحًا " .
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.'"
Reference : Sunan Ibn Majah 514In-book reference : Book 1, Hadith 248English translation : Vol. 1, Book 1, Hadith 514