Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
bayhaqi:751Abū al-Ḥusayn b. Bishrān> Ismāʿīl b. Muḥammad al-Ṣaffār > Saʿdān b. Naṣr > Muʿādh b. Muʿādh > Ashʿath > al-Ḥasan

[Machine] He said: If you are uncertain about performing ablution (wudu) but you are certain about your state of cleanliness, then you are on your wudu. And if you are uncertain about your cleanliness but you are certain about performing ablution, then perform ablution.  

البيهقي:٧٥١أَخْبَرَنَا أَبُو الْحُسَيْنِ بْنُ بِشْرَانَ بِبَغْدَادَ ثنا إِسْمَاعِيلُ بْنُ مُحَمَّدٍ الصَّفَّارُ ثنا سَعْدَانُ بْنُ نَصْرٍ ثنا مُعَاذُ بْنُ مُعَاذٍ عَنْ أَشْعَثَ عَنِ الْحَسَنِ

أَنَّهُ قَالَ إِذَا شَكَكْتَ فِي الْحَدَثِ وَأَيْقَنْتَ الْوُضُوءَ فَأَنْتَ عَلَى وُضُوئِكَ وَإِذَا شَكَكْتَ فِي الْوُضُوءِ وَأَيْقَنْتَ بِالْحَدَثِ فَتَوَضَّأْ