Musnad Ahmad 1065:18

Abdullah said: I found this hadith in my father’s book, in his own handwriting: Ishaq ibn Isa narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Amr ibn Abi Amr, from Asim ibn Umar ibn Qatadah, from Mahmud ibn Labid, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the thing I fear most for you is the minor shirk (polytheism)." They said: "O Messenger of Allah, and what is the minor shirk?" He said: "Showing off (al-riya')." Indeed, Allah says on the day when the servants are recompensed for their deeds: "Go to those whom you used to show off your deeds to in the world, and see if you find any reward with them."