"Those are the truthful ones," meaning: they have believed in what has come from Allah. "And the witnesses before their Lord," according to the interpretation of Mujahid: they testify to themselves [regarding] their faith in Allah.
Interpretation of Surat al-Hadid from verse 20 to verse 24.
﴿أُولَئِكَ هم الصديقون﴾ صدّقوا بِمَا جَاءَ من عِنْد الله ﴿وَالشُّهَدَاء عِنْد رَبهم﴾ تَفْسِير مُجَاهِد: يشْهدُونَ على أنفسهم بِالْإِيمَان بِاللَّه.
تَفْسِير سُورَة الْحَدِيد من الْآيَة ٢٠ إِلَى آيَة ٢٤.