What is said regarding the rule that the plaintiff has to produce a proof

The Statement of Allah تعالى: “O you who believe! When you contract a debt for a fixed period, write it down…” (V.2:282) And the Statement of Allah جل جلاله: “O you who believe! Stand out firmly for justice, as witnesses to Allah… (to the end of the verse) Ever Well-Acquainted with what you do.” (V.4:135)