Sahih Al Bukhari – Book of Judgments (Ahkaam)
Volumn 009, Book 089, Hadith Number 272.
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Narated By 'Abdur Rahman bin Abi Bakra : Abu Bakra wrote to his son who was in Sijistan: 'Do not judge between two persons when you are angry, for I heard the Prophet saying, "A judge should not judge between two persons while he is in an angry mood."