وَلَيْسَ عَلَى الْمُتَيَمِّمِ إِذَا لَمْ يَغْلِبْ عَلَى ظَنِّهِ أَنَّ بِقُرْبِهِ مَاءً أَنْ يَطْلُبَ الْمَاءَ، وَإِنْ غَلَبَ عَلَى ظَنِّهِ أَنَّ هُنَاكَ مَاءً: لَمْ يَجُزْ لَهُ أَنْ يَتَيَمَّمَ حَتَّى يَطْلُبَهُ.
Meaning: If one does not strongly suspect water nearby, searching is not mandatory; if one strongly suspects there is water, tayammum is not valid until one searches for it.
A dominant probability is stronger than a bare possibility. Nearby signs of water can make searching obligatory.