Sayyiduna Jarir (may Allah be pleased with him) narrates: Ali bin Abi Talib (may Allah be pleased with him) sent Ibn Abbas and Ash'ath bin Qays (may Allah be pleased with them both) to me while I was at Qirqisiya. They both said: The Commander of the Believers sends you his greetings of peace and says: Your separating from Mu'awiyah, which Allah has shown you to be better, is good, and I give you the same standing with me that the Messenger of Allah (peace and blessings be upon him) gave you. Sayyiduna Jarir (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) sent me to Yemen to fight them and to invite them to say that there is no true god except Allah. Once they said it, their blood and their wealth became inviolable, and I do not fight anyone who says, 'There is no true god except Allah.'" So we returned on that basis.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2379