Third:

677. If a person fears that if he uses water, his life will be endangered, or he will suffer from some ailment or physical defect, or the illness from which he is already suffering will be prolonged, or become acute or some complications may arise in its treatment, he should perform Tayammum. However, if he can avoid the harm by using warm water, he should prepare warm water and perform Wudhu, or Ghusl when it is necessary.

678. It is not necessary to be absolutely certain that water is harmful to him. If he feels that there is a probability of harm, and if that probability is justified by popular opinion, giving cause for some fear, then he should do Tayammum.

679. If a person has an eye disease and water is harmful to him he should perform Tayammum.

680. If a person performs Tayammum on account of certainty or fear about water being harmful to him but realizes before Namaz that it is not harmful, his Tayammum is void. And if he realizes this after having prayed and before the time ends, as per obligatory precaution, he should offer the prayers again with Wudhu or Ghusl. There is no Qadha after the time has ended.

681. If a person was sure that water was not harmful to him, and he performs Ghusl or Wudhu, but later realized that water was harmful to him, as per obligatory precaution, if there is still time and he has not offered his prayers then he should perform Tayammum too to offer his prayers. And if he has already offered his prayer and there is still time left then he should perform Tayammum and pray again. And if the time has ended and water is no longer harmful for him, then he should perform Wudhu or Ghusl, and perform the Qadha of the prayer he offered.