Musnad Ahmad 2:62

Abu Sa’id narrated to us, Daylam ibn Ghazwan, the ‘Abdi, narrated to us, Maymun al-Kurdi narrated to us, Abu ‘Uthman al-Nahdi narrated to me, from ‘Umar ibn al-Khattab, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, the thing I fear most for my nation is every hypocrite who is learned of tongue."