Nahjul Balagha Part 1, The Sermons

Sermon 169: There is no doubt that Allah sent down the Prophet (S) as a guide…

When the people of Jamal set off for Basrah Amir al-mu'minin said:

There is no doubt that Allah sent down the Prophet (S) as a guide with an eloquent Book and a standing command. No one will be ruined by it except one who ruins himself. Certainly, only doubtful innovations cause ruin except those from which Allah may protect. In Allah's authority lies the safety of your affairs. Therefore, render Him such obedience as is neither blameworthy nor insincere. By Allah, you must do so, otherwise Allah will take away from you the power of Islam, and will never thereafter return it to you till it reverts to others.

Certainly, these people are in agreement in disliking my authority. I will carry on till I perceive disunity among you; because if, in spite of the unsoundness of their view, they succeed, the whole organisation of the Muslims will be shattered. They are hankering after this world out of jealousy against him on whom Allah has bestowed it. So they intend reverting the matters on their backs (pre-Islamic period), while on us it is obligatory, for your sake, to abide by the Book of Allah (Qur'an), the Sublime, and the conduct of the Prophet of Allah, to stand by His rights and the revival of his sunnah.

Alternative Sources for Sermon 169

(1) Al-Tabari, Ta'rikh, VI, 163