ولو وجد ماء فأخبره عدل أنه نجس لا يجوز له التوضي به بل يتيمم
If water is available but a trustworthy and upright informant establishes that it is impure, it may not be used for ablution in this example; tayammum is performed instead.
Visible availability of water is not the same as its legal suitability. A mere thought that the water looks usable does not override legally recognized testimony about its impurity.
The illustration depends on a qualifying report and its legal consequence. Other questions—such as the availability of different water—are separate. The issue here is the order of recognized evidence and personal estimation.