Masruq (may Allah have mercy on him) said: Abdullah (may Allah be pleased with him) was asked about suht (unlawful gain), and he said: "It is bribes." The questioner said: "In judgement?" Abdullah (may Allah be pleased with him) said: "That is disbelief." Then he recited this verse: "And whoever does not judge by what Allah has revealed, then it is they who are the disbelievers." [Surah al-Ma'idah: 44]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20480