Anyone who is unable to use water due to illness, is permitted to perform Tayammum even if water is available nearby. However, it should be kept in mind that the illness should be the kind that will be adversely affected if the patient uses water; this is something that can be known through experimentation by the patient himself or by consulting a trustworthy, specialized doctor.
The eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, states: “One of the dispensations that allows one to perform Tayammum is when the use of water is likely to cause one harm. Tayammum is lawful in this case even if there is available water. There is a tenet in Islam to the effect that there should be neither harm nor reciprocating harm. As well as this, Allah, the Almighty has not laid upon His worshippers any hardship in religion.
The Qur’anic verses that refer to the dispensation of Tayammum mention illness as one of the factors that allow one to perform Tayammum as a substitution for ablution (or taking a purifying bath).
Allah, the Almighty says: ‘O ye who believe! Approach not prayers with a mind befogged, until ye can understand all that ye say,- nor in a state of ceremonial impurity (Except when traveling on the road), until after washing your whole body. If you are ill, or on a journey, or one of you comes from the offices of nature, or you have been in contact with women, and you find no water, then take for yourselves clean sand or earth, and rub therewith your faces and hands. For Allah doth blot out sins and forgive again and again.’ (An-Nisa’: 43)
He also says: ‘O ye who believe! When you prepare for prayer, wash your faces, and your hands (and arms) to the elbows; Rub your heads (with water); and (wash) your feet to the ankles. If you are in a state of ceremonial impurity, bathe your whole body. But if ye are ill, or on a journey, or one of you comes from offices of nature, or you have been in contact with women, and you find no water, then take for yourselves clean sand or earth, and rub therewith your faces and hands, Allah doth not wish to place you in a difficulty, but to make you clean, and to complete His favour to you, that you may be grateful.’ (Al-Ma’dah: 6)
Hence, a patient may use Tayammum as a substitution for ablution or taking a purifying bath when the use of water is expected to cause him harm, or worsen his wound, or delay his cure, or increase his pain. A healthy person may even resort to Tayammum if he is afraid that water may harm him if used in very cold weather.
There is evidence for this. It was reported that when the Prophet (peace and blessings be upon him) appointed `Amr ibn Al-`As as a leader of the Muslim army sent to the battle of Dhat Asalasel, `Amr performed Tayammum when it was cold and then led his companions in prayer. When the army returned home, some of them told the Prophet (peace and blessings be upon him) about what `Amr had done. The Prophet (peace and blessings be upon him) said to `Amr: ‘O `Amr, did you lead the people in prayer while you were in a state of major impurity?’ `Amr said: ‘I remembered Almighty Allah’s words: ‘O ye who believe! Eat not up your property among yourselves in vanities: But let there be amongst you Traffic and trade by mutual good-will: Nor kill (or destroy) yourselves: for verily Allah hath been to you Most Merciful!’ (An-Nisa: 29) So, I performed Tayammum and prayed.’ [Hearing that,] the Prophet (peace and blessings be upon him) smiled and did not comment. [`Amr regarded Almighty Allah’s words ‘Nor kill (or destroy) yourselves’ as a dispensation for him in that case to perform Tayammum, as he was afraid that using water on that very cold night might cause his death.]
Furthermore, Abu Dawud in his book Sunnan Abu Dawud, reported under the Chapter of ‘The Wounded Can Perform Tayammum‘ that the Prophet’s companion Jabir (may Allah be pleased with him) said: ‘Once, I went on a journey with some of my companions. One of us was hit by a stone, and thus received a (severe) wound in his head. Then, he had a wet dream. Upon that he asked us if there was a dispensation for him to perform Tayammum as he had been severely wounded. He was answered: ‘We do not find a dispensation for you while there is available water (that you can use for washing).’ Because of these words, he took a bath, but he died. When the Prophet (peace and blessings be upon him) was told about that, he (peace and blessings be upon him) was angry and said: ‘They [those who told the man he could not perform Tayammum] have killed him. May Allah, the Almighty kill them! Why did they not ask (a knowledgeable person) before they gave an answer to something they did not know about? The cure of ignorance lies in seeking knowledge. It would have been sufficient for him to perform Tayammum or wrap his wound with a piece of cloth and wipe over it with water, while washing the rest of his body.’ “