Reference for Makkot 22:11

מאי הוה ליה למעבד היה לו לבקש רחמים שיגמור דינו לזכות ולא ביקש

What should [this high priest] have done [he was appointed after the murder]? He should have beseeched [God] for mercy that his judgement should find a merit, and he didn't beseech.

Jastrow