Hadith on Shirk: Hidden idolatry is ostentation, showing off

Abu Sa’id al-Khudri reported: The Messenger of Allah, peace and blessings be upon him, came to us while we were discussing the False Messiah. The Prophet said, “Shall I not tell you about my greater fear for you than the False Messiah?” We said, “Of course!” The Prophet said, “It is hidden idolatry, that a man stands for prayer and beautifies his prayer when he sees another man looking at him.”

Source: Sunan Ibn Mājah 4204

Grade: Hasan (fair) according to Al-Albani

عَنْ أَبِي سَعِيدٍ قَالَ خَرَجَ عَلَيْنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَنَحْنُ نَتَذَاكَرُ الْمَسِيحَ الدَّجَّالَ فَقَالَ أَلَا أُخْبِرُكُمْ بِمَا هُوَ أَخْوَفُ عَلَيْكُمْ عِنْدِي مِنْ الْمَسِيحِ الدَّجَّالِ قَالَ قُلْنَا بَلَى فَقَالَ الشِّرْكُ الْخَفِيُّ أَنْ يَقُومَ الرَّجُلُ يُصَلِّي فَيُزَيِّنُ صَلَاتَهُ لِمَا يَرَى مِنْ نَظَرِ رَجُلٍ

4204 سنن ابن ماجه كتاب الزهد باب الرياء والسمعة

3408 المحدث الألباني خلاصة حكم المحدث حسن في صحيح ابن ماجه