وَعَنْ مَحْمُودِ بْنِ لَبِيدٍ رضى الله عنه
قَالَ: قَالَ رَسُولُ اَللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ -«إِنَّ أَخْوَفَ مَا أَخَافُ عَلَيْكُمْ اَلشِّرْكُ اَلْأَصْغَرُ: اَلرِّيَاءُ»
“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.