I heard the Messenger of Allah ﷺ say, "Reconciliation between Muslims is permissible, except a reconciliation that forbids what is lawful or permits what is forbidden."
حدثنا أبو بكر بن أبي شيبة حدثنا خالد بن مخلد حدثنا
سمعت رسول الله ﷺ يقول الصلح جائز بين المسلمين إلا صلحا حرم حلالا أو أحل حراما
Reference. [sahih] Tirmidhi narrated it in the Book of Rulings, chapter on what was mentioned from the Messenger of Allah ﷺ concerning reconciliation between people, hadith no. 1352, from the hadith of Kathir with this wording, and he said it is hasan sahih.Dhahabi said in Mizan al-Itidal 3/407, "As for Tirmidhi, he narrated from his hadith, 'reconciliation is permissible between Muslims,' and graded it sahih. For this reason the scholars do not rely on Tirmidhi's grading of sahih. See hadith no. 165 for its defect."But Kathir was not the only one who narrated it. Abu Dawud narrated it, hadith no. 3594, from the hadith of Walid bin Rabah, from Abu Hurairah, with the same wording, and its chain of transmission is hasan. Ibn Jarud graded it sahih, hadith nos. 637 and 638, as did Ibn Hibban in Mawarid, hadith no. 1199.