The ruling on one who performed tayammum when he began the prayer and then found water

A group in the desert performed tayammum and wanted to pray, but a man brought them water after the iqamah had been called?
There is scholarly disagreement on this. If a person finds water after he has begun the prayer, is he required to break off the prayer and use the water, or does he continue? The scholars have two positions on this, and the precaution is to break off the prayer and perform wudu with the water.