(c) It is narrated from al-Hasan (may Allah have mercy on him) that when you sense that you may have broken your ablution but are certain of your ablution, then you are still in a state of ablution; and when you are in doubt about your ablution but are certain of having broken it, then perform ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 751