Quran 5:107 — The Feast (Al-Ma'edah)

If the witnesses are found to be guilty of bias, then two others shall be asked to take their places. Choose two persons who were victimized by the first witnesses, and let them swear by GOD: "Our testimony is more truthful than theirs; we will not be biased. Otherwise, we will be transgressors."

[5:106] O you who believe, witnessing a will when one of you is dying shall be done by two equitable people among you. If you are traveling, then two others may do the witnessing. After observing the Contact Prayer (Salat), let the witnesses swear by GOD, to alleviate your doubts: "We will not use this to attain personal gains, even if the testator is related to us. Nor will we conceal GOD's testimony. Otherwise, we would be sinners."

[5:108] This is more apt to encourage an honest testimony on their part, fearing that their oath may be disregarded like that of the previous witnesses. You shall observe GOD and listen. GOD does not guide the wicked.