Surah Al Ma'idah Tafseer

فَإِنْ عُثِرَ عَلَى أَنَّهُمَا اسْتَحَقَّا إِثْمًا فَأخَرَانِ يِقُومَانُ مَقَامَهُمَا مِنَ الَّذِينَ اسْتَحَقَّ عَلَيْهِمُ الَاوْلَيَانِ

If it then becomes known that these two had been guilty of sin, let two others stand forth in their places, nearest in kin from among those who claim a lawful right.

فَيُقْسِمَانِ بِاللّهِ لَشَهَادَتُنَا أَحَقُّ مِن شَهَادَتِهِمَا

Let them swear by Allah (saying):"We affirm that our testimony is truer than that of both of them..."

Meaning, our testimony that they have cheated is more truthful than the testimony that they have offered,

وَمَا اعْتَدَيْنَا

and that we have not trespassed (the truth),

when we accused them of treachery,

إِنَّا إِذًا لَّمِنَ الظَّالِمِينَ

for then indeed we should be of the wrongdoers.

if we had lied about them.

This is the oath of the heirs, and preference is to be given to their saying. Just as in the case with the oath of relative of a murdered person if he attempts to tarnish the case of the murdered person. So his family takes an oath in defense of his honor. This is discussed in the studies of the oaths in the books of Ahkam.

Allah's statement