Mahmud bin Labid (RAA) narrated that the Messenger of Allah (ﷺ) said:
"The thing I fear most for you is the lesser shirk (polytheism), showing-off (of good deeds)." Related by Ahmad with a good chain of narrators, (DE)
English reference (DE): Book 16, Hadith 1527 (DE)
Arabic reference (DE): Book 16, Hadith 1484 (DE)