Truly pious people tried their best to reject the office of the qadi when it was offered to them by the ruler. They even pretended to be blind or prefered to be imprisoned - so much did they fear even getting close to issuing injuste rulings.
The strongest manifest for absolute justice is in the Qur'an itself, where Allah commands:
"O ye who believe! Stand out firmly for justice, as witnesses to Allah, even as against yourselves, or your parents, or your kin, and whether it be (against) rich or poor: for Allah can best protect both. Follow not the lusts (of your hearts), lest ye swerve, and if ye distort (justice) or decline to do justice, verily Allah is well-acquainted with all that ye do."
(Sura an-Nisa 4 : 135, Translation of Abdullah Yusuf Ali, 1985)