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."