Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "What I fear most for my Ummah are three things: the slip of a scholar, the hypocrite's disputing by means of the Qur'an, and the world that will be opened up to you."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17418