Question: Should a prisoner and an ill person who is unable to make tayammum perform salat?
ANSWER
If an ill person who is unable to perform wudu’ or ghusl cannot find anyone to help him even for money, he should make tayammum. He who cannot make tayammum even with help does not perform salat. He has to make up his missed salats when he recovers.

A prisoner who cannot find a clean place to perform salat, water, or soil should pretend to be performing salat without reciting anything. He will have to make up all these salats when he is set free.

The difference between these two people is this: The ill person cannot even perform tayammum. The prisoner has the strength to perform tayammum and salat, but he has neither water to make wudu’ nor substances to make tayammum. The former is ill while the latter is healthy. A healthy person must do something to perform salat. For this reason, he should pretend to be performing salat. But he will make up these salats when he is set free.

That a prisoner, though he has an excuse, should look for a way-out and pretend to be performing salat is a good example that shows the importance of salat.

As for the ill person, it is permissible for him to miss a salat because he is unable to make even tayammum. If he recovers, he will have to make up all his missed salats. If he does not recover, they will be forgiven.