Salamah narrates that Sayyidah Umm Salamah (may Allah be pleased with her) said: Sayyiduna al-Zubayr (may Allah be pleased with him) had a dispute with a man before the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) judged in favour of Sayyiduna al-Zubayr (may Allah be pleased with him). The man said: He (peace and blessings be upon him) only judged in his favour because he is the son of his paternal aunt. So this verse was revealed: "But no, by your Lord, they will not be believers until they make you judge" [Surah al-Nisa': 65].
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19956