Musnad Ahmad 1065:13

Ibrahim ibn Abi al-Abbas narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Amr ibn Abi Amr, from Asim ibn Umar al-Zafari, from Mahmud ibn Labid, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the thing I fear most for you..." and he mentioned its meaning.