Because Allah the Exalted has said: "When any of you comes from relieving himself and you do not find water, then perform tayammum." ‘Ata says that if something like a worm or a louse comes out from a person’s back passage (i.e., anus) or front passage (i.e., penis or vagina), he should repeat his ablution. And Jabir bin ‘Abdullah says that if a person laughs during prayer, he should repeat the prayer but not the ablution. And Hasan (al-Basri) said that whoever shaves his hair, cuts his nails, or removes his socks (after ablution), there is no need for him to repeat ablution. Abu Huraira (may Allah be pleased with him) says that ablution is not obligatory except due to hadath (i.e., a state of ritual impurity). And it is narrated from Jabir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was present during the battle of Dhat ar-Riqa’. A man was struck by an arrow and much blood flowed from his body, yet he still performed bowing and prostration and completed his prayer. And Hasan al-Basri said that Muslims used to always pray despite their wounds. And according to Tawus, Muhammad bin ‘Ali, and the people of Hijaz, ablution is not obligatory due to (the flow of) blood. ‘Abdullah bin ‘Umar (may Allah be pleased with them both) pressed a boil of his and blood came out, but he did not repeat his ablution. And Ibn Abi Awfa spat blood, yet he continued his prayer. And Ibn ‘Umar and Hasan (may Allah be pleased with them) said regarding the one who is cupped, that he should wash the place where he was cupped, and there is no need to repeat ablution.

Narrated Abu Huraira: Allah's Apostle said, "A person is considered in prayer as long as he is waiting for the prayer in the mosque as long as he does not do Hadath." A non-Arab man asked, "O Abu Huraira! What is Hadath?" I replied, "It is the passing of wind (from the anus) (that is one of the types of Hadath).

Narrated `Abbad bin Tamim: My uncle said: The Prophet said, "One should not leave his prayer unless he hears sound or smells something."

Narrated `Ali: I used to get emotional urethral discharges frequently and felt shy to ask Allah's Apostle about it. So I requested Al-Miqdad bin Al-Aswad to ask (the Prophet) about it. Al-Miqdad asked him and he replied, "On has to perform ablution (after it).

Narrated Zaid bin Khalid: I asked `Uthman bin `Affan about a person who engaged in intercourse but did no discharge. `Uthman replied, "He should perform ablution like the one for an ordinary prayer but he must wash his penis." `Uthman added, "I heard it from Allah's Apostle." I asked `Ali Az-Zubair, Talha and Ubai bin Ka`b about it and they, too, gave the same reply. (This order was canceled later on and taking a bath became necessary for such cases).

Narrated Abu Sa`id Al-Khudri: Allah's Apostle sent for a Ansari man who came with water dropping from his head. The Prophet said, "Perhaps we have forced you to hurry up, haven't we?" The Ansari replied, "Yes." Allah's Apostle further said, "If you are forced to hurry up (during intercourse) or you do not discharge then ablution is due on you (This order was canceled later on, i.e. one has to take a bath).

Narrated Usama bin Zaid: "When Allah's Apostle departed from `Arafat, he turned towards a mountain pass where he answered the call of nature. (After he had finished) I poured water and he performed ablution and then I said to him, "O Allah's Apostle! Will you offer the prayer?" He replied, "The Musalla (place of the prayer) is ahead of you (in Al-Muzdalifa).

Narrated Al-Mughira bin Shu`ba: I was in the company of Allah's Apostle on one of the journeys and he went out to answer the call of nature (and after he finished) I poured water and he performed ablution; he washed his face, forearms and passed his wet hand over his head and over the two Khuff (socks made from thick fabric or leather).

Mansur narrated from Ibrahim that there is no harm in reciting the Qur'an in the bathhouse (hammam), and likewise, there is no harm in writing letters without ablution. And Hammad narrated from Ibrahim that if the person (in the bathhouse) has a waist-wrapper on his body, then greet him with salam, and if he does not (have a waist-wrapper), then do not greet him.

Narrated `Abdullah bin `Abbas: That he stayed overnight in the house of Maimuna the wife of the Prophet, his aunt. He added: I lay on the bed (cushion transversally) while Allah's Apostle and his wife lay in the lengthwise direction of the cushion. Allah's Apostle slept till the middle of the night, either a bit before or a bit after it and then woke up, rubbing the traces of sleep off his face with his hands. He then, recited the last ten verses of Sura Al-`Imran, got up and went to a hanging water-skin. He then Performed the ablution from it and it was a perfect ablution, and then stood up to offer the prayer. I, too, got up and did as the Prophet had done. Then I went and stood by his side. He placed his right hand on my head and caught my right ear and twisted it. He prayed two rak`at then two rak`at and two rak`at and then two rak`at and then two rak`at and then two rak`at (separately six times), and finally one rak`a (the witr). Then he lay down again in the bed till the Mu'adh-dhin came to him where upon the Prophet got up, offered a two light rak`at prayer and went out and led the Fajr prayer.