Is it obligatory for one who has performed ablution to repeat it if someone tells him that it has been nullified, even if there is an indication supporting the truthfulness of what he says, or is the original principle to remain in a state of purity?

The fundamental principle is that certainty is not removed by doubt, and a singular report (khabar al-wahid) does not provide certainty; thus, the original state of purity remains presumed. Some scholars have held that the report of a trustworthy person is acceptable in this situation. The Shafi'is discussed the issue: if one slept a sleep that does not invalidate ablution, and then a trustworthy person informs him that his ablution has been invalidated, is he obliged to accept this report? Ibn Hajar al-Haytami favored the view that he is obliged to accept the report, explaining that this presumption (dhann) is considered legally equivalent to certainty in many chapters of jurisprudence. Others held that it does not invalidate ablution based on the reports of trustworthy individuals, because their report provides mere presumption, and the certainty of purity is not lifted by the presumption of its opposite. The more cautious approach is to perform ablution in this situation to absolve oneself and act upon the more precautious opinion, especially if there is an indication supporting the veracity of the report.