Abu Harb bin al-Aswad al-Dili narrated from his father that Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) "would not host a party to a dispute unless the other party was present."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20472