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 most important criterion of belief:
The Cattle/6:82
Those who believe and do not mix their faith with wrongdoing, they deserve security and they have found the truth.
Luqman/31:13
"When Lokman advised his son, he said: "My dear son, do not associate partners with Allah; surely shirk (associating partners with God) is a great wrong."
The Companies/39:65
It has been revealed to you and those before you that if you associate partners, all that you do will be in vain and you will be of the losers.