Ali bin Abi Talhah narrates from Ibn Abbas (may Allah be pleased with them both) concerning this saying of Allah Almighty: "And let the witnesses not refuse when they are called" [Surah al-Baqarah: 282], that he said: When the Muslims need his testimony, he must not refuse to testify. After this He said: "And let no scribe or witness be harmed" [Surah al-Baqarah: 282].
Harming is that a man says to another, while he has no need of him: Allah has commanded you not to refuse when you are called to testify. He thus causes him hardship, although another would suffice in his place; so Allah forbade this: "And if you do so, you will be sinners" [Surah al-Baqarah: 282]. By fusuq is meant disobedience.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20603