Idris al-Awdi (may Allah have mercy on him) said that Sa'id bin Abi Burdah (may Allah have mercy on him) brought out a document and said: This is the letter that Sayyiduna Umar (may Allah be pleased with him) sent to Sayyiduna Abu Musa (may Allah be pleased with him). In it was written: "Reconciliation between people is permissible, except a reconciliation that makes the unlawful lawful and the lawful unlawful; that is not permissible."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلح / 11353