Because Allah, the Exalted, has said (in Surah Al-Baqarah):“O you who believe! When you contract a debt for a fixed term, write it down. And let a scribe write it down between you in justice. And the scribe should not refuse to write as Allah has taught him, so let him write. And let the one upon whom the obligation is dictate, and let him fear Allah, his Lord, and not diminish anything from it. But if the one upon whom the obligation is, is of limited understanding or weak or unable to dictate himself, then let his guardian dictate in justice. And bring to witness two witnesses from among your men. And if there are not two men, then a man and two women from those whom you accept as witnesses, so that if one of the two women errs, then the other can remind her. And let not the witnesses refuse when they are called upon. And do not be weary to write it, whether it is small or large, for its term. That is more just in the sight of Allah and stronger as evidence and more likely to prevent doubt between you, except when it is an immediate transaction which you conduct among yourselves, then there is no blame upon you if you do not write it. And take witnesses when you make a commercial contract. And let neither scribe nor witness be harmed. For if you do so, indeed, it is grave disobedience in you. And fear Allah. And Allah teaches you. And Allah is Knowing of all things.”__QUR_2:282__

And the statement of Allah, the Exalted:“O you who believe! Stand out firmly for justice, as witnesses to Allah, even if it be against yourselves or your parents or your relatives, whether one is rich or poor, Allah is more worthy of both. So do not follow the desires (of your hearts), lest you avoid justice. And if you distort (your testimony) or refuse (to give it), then indeed Allah is ever, with what you do, Well-Acquainted.”__QUR_4:135__

Narrated `Urwa bin Al-Musaiyab Alqama bin Waqqas and Ubaidullah bin `Abdullah: About the story of `Aisha and their narrations were similar attesting each other, when the liars said what they invented about `Aisha, and the Divine Inspiration was delayed, Allah's Apostle sent for `Ali and Usama to consult them in divorcing his wife (i.e. `Aisha). Usama said, "Keep your wife, as we know nothing about her except good." Buraira said, "I cannot accuse her of any defect except that she is still a young girl who sleeps, neglecting her family's dough which the domestic goats come to eat (i.e. she was too simpleminded to deceive her husband)." Allah's Apostle said, "Who can help me to take revenge over the man who has harmed me by defaming the reputation of my family? By Allah, I have not known about my family-anything except good, and they mentioned (i.e. accused) a man about whom I did not know anything except good."

And Amr bin Harith, may Allah be pleased with him, considered it permissible and said: "Such a situation can be adopted with a lying hypocrite." Shu'bi, Ibn Sirin, Ata, and Qatadah said that whoever hears something from someone can testify to it, even if he was not made a witness. And Hasan al-Basri, may Allah have mercy on him, said that he should say in this way: "Although those people did not make me a witness, I heard it in this manner."

Narrated `Abdullah bin `Umar: Allah's Apostle and Ubai bin Ka`b Al-Ansari went to the garden where Ibn Saiyad used to live. When Allah's Apostle entered (the garden), he (i.e. Allah's Apostle) started hiding himself behind the date108 palms as he wanted to hear secretly the talk of Ibn Saiyad before the latter saw him. Ibn Saiyad wrapped with a soft decorated sheet was lying on his bed murmuring. Ibn Saiyad's mother saw the Prophet hiding behind the stems of the date-palms. She addressed Ibn Saiyad saying, "O Saf, this is Muhammad." Hearing that Ibn Saiyad stopped murmuring (or got cautious), the Prophet said, "If she had left him undisturbed, he would have revealed his reality." (See Hadith No. 290, Vol 4 for details)

Narrated Aisha: The wife of Rifa`a Al-Qurazi came to the Prophet and said, "I was Rifa`a's wife, but he divorced me and it was a final irrevocable divorce. Then I married `Abdur-Rahman bin Az-Zubair but he is impotent." The Prophet asked her 'Do you want to remarry Rifa`a? You cannot unless you had a complete sexual relation with your present husband." Abu Bakr was sitting with Allah's Apostle and Khalid bin Sa`id bin Al-`As was at the door waiting to be admitted. He said, "O Abu Bakr! Do you hear what this (woman) is revealing frankly before the Prophet ?"

"And if other people say, 'We do not know anything about this matter,' then the judgment will be according to the statement of the one who testified in affirmation."

Al-Humaydi said that this is like when Bilal (may Allah be pleased with him) informed that the Prophet (peace and blessings of Allah be upon him) prayed inside the Ka'bah, and Fadl (may Allah be pleased with him) said that he (peace be upon him) did not pray (inside the Ka'bah). So all the people accepted the testimony of Bilal (may Allah be pleased with him). Similarly, if two witnesses testify that so-and-so owes so-and-so one thousand dirhams, and another two witnesses testify that it is one and a half thousand dirhams, then the judgment will be according to the statement of those who testified to the greater amount.

Narrated `Abdullah bin Abu Mulaika from `Uqba bin Al-Harith: `Uqba married the daughter of Abu Ihab bin `Aziz, and then a woman came and said, "I suckled `Uqba and his wife." `Uqba said to her, "I do not know that you have suckled me, and you did not inform me." He then sent someone to the house of Abu Ihab to inquire about that but they did not know that she had suckled their daughter. Then `Uqba went to the Prophet in Medina and asked him about it. The Prophet said to him, "How (can you keep your wife) after it has been said (that both of you were suckled by the same woman)?" So, he divorced her and she was married to another (husband).

And Allah the Exalted said (in Surah At-Talaq) «وأشهدوا ذوى عدل منكم‏» "And take two just men from among you as witnesses." And (Allah the Exalted said in Surah Al-Baqarah) «ممن ترضون من الشهداء‏» "From among the witnesses whom you accept."

Narrated `Umar bin Al-Khattab: People were (sometimes) judged by the revealing of a Divine Inspiration during the lifetime of Allah's Apostle but now there is no longer any more (new revelation). Now we judge you by the deeds you practice publicly, so we will trust and favor the one who does good deeds in front of us, and we will not call him to account about what he is really doing in secret, for Allah will judge him for that; but we will not trust or believe the one who presents to us with an evil deed even if he claims that his intentions were good.