The Cow - 2:282
Believers! When you lend each other money for a certain term, write it down. Let a scribe from among you write it down with justice. Let not the scribe refrain from writing as Allah has taught him. And let the borrower dictate it. Let him fear Allah, his Lord, and not be dishonest in it. If the borrower is incompetent, or helpless, or incapable of dictating, his guardian should dictate with justice. And let two witnesses from your men bear witness. If you cannot find two male witnesses, then choose a man and two women from among the witnesses of your choice, so that if one of the women is mistaken, the other may remind her, and the witnesses should not hesitate when they are called. Do not hesitate to write it down, whether it is a little or a lot, with the date of payment. This is more just in the sight of Allah, and it is better for testimony, and it is more suitable so that you may not be in doubt. But if the trade is in cash, there is no harm in not writing it down. Let there be witnesses when you trade. Do not harm either the writer or the witness. Otherwise you will do evil to yourselves. Listen to Allah. Allah is teaching you. Allah knows everything.
The testimony of the believers:
The Cow/2:177
It is not good that you turn your faces towards the east or the west. The righteous are those who believe in Allah, the Last Day, the angels, the Book and the Prophets, and they give freely to relatives, orphans, the needy, the stranded, the beggars, and to free the slaves, and they observe the prayer, and pay the zakat, and keep their word when they contract, and they resist hardship, distress and oppression. It is they who are the righteous, and it is they who are the virtuous.
The Cattle/6:152
Do not approach the property of orphans until they reach the age of maturity. Measure and weigh accurately. We do not burden a person with what he is not able. When you speak, be truthful even against your relatives. Keep your promise to Allah. These are His admonitions to you that you may take a lesson.
The Women/4:135
Believers! Bear witness for the sake of Allah and observe justice, even against yourselves and your parents and your relatives. Whether rich or poor, Allah takes care of both groups. So do not be biased in accordance with your personal interests and feelings. If you distort the truth, or if you refrain from testifying, know that Allah is aware of what you do.
The Criterion/25:72
They do not bear false witness. When they are confronted with empty words, they ignore them and pass by.
The Coalition/33:70
O you who believe, honor Allah and speak the truth.
The Man/76:7
They fulfill their promises and fear a Day that is exceedingly evil.
The Ways of Ascent/70:33
They bear testimony properly;