Sayyiduna Awf bin Malik (may Allah be pleased with him) and Ibn Abd Kulal entered the mosque of Hims and saw a gathering. Awf (may Allah be pleased with him) asked: What is this? The people said: Ka'b is preaching. He said: Woe to him! Has he not heard this hadith of the Messenger of Allah (peace and blessings be upon him): "None should preach to the people except a ruler, or one appointed by the ruler, or a vain person."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15191