فَلَمْ تَجِدُوا مَاءً فَتَيَمَّمُوا
If you cannot find water, then perform tayammum.
Recognition of tayammum as purification does not mean that it remains sufficient after water is available and can be used. The conditions establishing a ruling must be distinguished from the conditions terminating it.
In the Hanafi view, valid tayammum confers ritual purity until an invalidating circumstance occurs. Ability to use water can be such a circumstance. “Tayammum purifies” and “tayammum can never expire” are not equivalent claims.