Kathir bin Abdullah narrates from his father, from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "Reconciliation between Muslims is permissible, except a reconciliation that makes the unlawful lawful and the lawful unlawful."
Abu Amir al-Aqadi also narrated it likewise from Kathir bin Abdullah, and reliance is on his narration, for Muhammad bin al-Hasan bin Zabalah is utterly da'if (weak); and the narration of Kathir bin Abdullah bin Amr bin Awf al-Muzani, when joined to the one before it, the two strengthen each other.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلح / 11352