No ablution except for impurity Sunan Ibn Majah 513
Arabic hadith text
حَدَّثَنَا مُحَمَّدُ بْنُ الصَّبَّاحِ، قَالَ أَنْبَأَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ سَعِيدٍ، وَعَبَّادِ بْنِ تَمِيمٍ، عَنْ عَمِّهِ، قَالَشُكِيَ إِلَى النَّبِيِّ ـ صلى الله عليه وسلم ـ الرَّجُلُ يَجِدُ الشَّىْءَ فِي الصَّلاَةِ فَقَالَ " لاَ حَتَّى يَجِدَ رِيحًا أَوْ يَسْمَعَ صَوْتًا "
Translation
'Abbad bin Tamim narrated that his paternal uncle said
"A complaint was made to the Prophet about a man who sensed something (some doubt about his ablution) during prayer. He said: 'No (he does not have to perform ablution) unless he notices a smell or hears a sound.'"