Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
You can do tayammum: 1- when you cannot find water, or the amount you find is insufficient for wudu, 2- when you are injured or ill, 3- if the water is cold enough to physically harm you, 4- when water is nearby, but you do not want to fetch it due to fear, 50 If you are saving his water for later use, 6- you can get water, but fear that the prayer will be over by the time you get it.
In his famous work, Fiqh As-Sunnah, Sheikh Sayyid Sabiq, elaborates on the subject as follows:
When can you do tayammum?
These are the case when it is permissible for a Muslim to do tayammum instead of wudu:
- When you cannot find water, or the amount you find is insufficient for ablution
`Imran bin Husain said, “We were with the Messenger of Allah (peace be upon him) during a journey. When he led the people in prayer, one man stayed apart. He asked him, “What prevented you from praying?” He said, ‘I need a post-nocturnal bath and there is no water.’ He said, ‘Use the soil, for it is sufficient.”’ (Al-Bukhari and Muslim)
Abu Zhar` related that the Prophet (peace be upon him) said, “The soil is a purifier for a Muslim, even if he does not find water for twenty years.” (Related by “the four” At-Tirmidhi grades it hasan sahih.)
But before one makes tayammum, he must look for water from any possible source. If he is sure water is not to be found or it is too far away, he does not have to look for it.
- When you are injured or ill
If one is in this condition, and believes water will worsen it (he does not have to be absolutely sure, but may base his opinion on past experience or what a knowledgeable person has told him), he may perform tayammum.
Jabir said, “We were on a journey and one of us got injured. Later, he had a wet dream. He asked his companions, ‘Can I perform tayammum?’ They said, ‘No, not if you have water.’ He performed ghusl and died. When they came to the Messenger of Allah, they informed him of what had transpired. He said, ‘They killed him, Allah will kill them. Do you not ask if you do not know?
The rescue of the ignorant person is the question. He could have performed tayammum and dropped water on his wound or wrapped it with something and wipe over the wrapping, and wash the rest of his body.” This is related by Abu Dawud, Ibn Majah, ad-Daraqutni and Ibn as-Sakin, who said it is sahih.
- If the water is cold enough to physically harm the user.
This is only allowed on the condition that he can find no one to heat it, or is unable to use the public bathrooms.
`Amr ibn al-`Aas narrated that he was participating in an expedition. He had a wet dream during an extremely cold night, and was afraid that if he performed ghusl he would die. He prayed the morning prayer with his companions. He then went to the Messenger of Allah (peace be upon him) to ask him about this. Prophet Muhammad said, “O `Amr, did you pray with your companions while you needed a post-nocturnal bath?” `Amr mentioned the verse, “Do not kill yourselves, Allah is merciful to you” to the Prophet. The Prophet just laughed and didn’t say anything. (Related by Ahmad, Abu Dawud, al-Hakim, ad-Daraqutni, Ibn Hibban and al-Bukhari in mu`allaq form.) This example illustrated the Prophet’s tacit approval.
- When water is nearby, but you do not want to fetch it due to fear.
If one fears for his life, family, wealth, (for example, if an enemy is nearby–beast or human–or one is a prisoner, and so on), one may perform tayammum. This is also allowed if there is water but one lacks the proper means to get it, or if one fears some accusation against him if he gets it.
- If you are saving his water for later use.
This could be for a hound, for dough, cooking or to remove an impurity that is not pardonable. Says Imam Ahmad, “Many of the companions performed tayammum to save their water for drinking.” `Ali said that a man who is travelling and becomes unclean because of sex or a wet dream can perform tayammum if he fears he will go thirsty: “He should perform tayammum and not ghusl.” (Related by ad-Daraqutni.)
Ibn Taimiyyah said: “If a person needs to relieve himself but has only a small amount of water, it is best that he pray with tayammum and relieve himself, rather than keep his ablution and pray before relieving himself.”
- When you can get water, but fear that the prayer will be over by the time he gets it.
He can perform tayammum and pray, and does not need to repeat his prayer (after he gets water).
The soil used for tayammum
It must be pure soil: this can be sand, stone, gypsum, and so on. Allah says: (“Perform tyammum with pure soil,} and all scholars of Arabic agree that “soil” is whatever covers the earth, dirt or otherwise.
Almighty Allah knows best.