Sunan Ibn Majah, Hadith 4204 ⓘ
It was narrated that Abu Sa’eed said: “The Messenger of Allah came out to us when we were discussing Dajjal (False Christ) and said: ‘Shall I not tell you of that which I fear more for you than Dajjal?’ We said: ‘Yes.’ He said: ‘Hidden polytheism, when a man stands to pray and makes it look good because he sees a man looking at him.’”
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ سَعِيدٍ، حَدَّثَنَا أَبُو خَالِدٍ الأَحْمَرُ، عَنْ كَثِيرِ بْنِ زَيْدٍ، عَنْ رُبَيْحِ بْنِ عَبْدِ الرَّحْمَنِ بْنِ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنْ أَبِيهِ، عَنْ أَبِي سَعِيدٍ، قَالَ خَرَجَ عَلَيْنَا رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ وَنَحْنُ نَتَذَاكَرُ الْمَسِيحَ الدَّجَّالَ فَقَالَ " أَلاَ أُخْبِرُكُمْ بِمَا هُوَ أَخْوَفُ عَلَيْكُمْ عِنْدِي مِنَ الْمَسِيحِ الدَّجَّالِ " . قَالَ قُلْنَا بَلَى . فَقَالَ " الشِّرْكُ الْخَفِيُّ أَنْ يَقُومَ الرَّجُلُ يُصَلِّي فَيُزَيِّنُ صَلاَتَهُ لِمَا يَرَى مِنْ نَظَرِ رَجُلٍ " .
also referenced under: Book 37, Hadith 4344 (In-Book) & Volume 5, Book 37, Hadith 4204 (DarusSalam)