Musnad Ahmad 16498

Shall I tell you what I fear more for you than the Antichrist? They said: Yes, O Messenger of Allah. He said: Hidden polytheism; when a man gets up to pray and improves his prayer because he sees another man watching him

Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Shall I tell you what I fear more for you than the Antichrist?" They said: "Yes, O Messenger of Allah." He said: "Hidden polytheism; when a man gets up to pray and…

"‏ أَلاَ أُخْبِرُكُمْ بِمَا هُوَ أَخْوَفُ عَلَيْكُمْ عِنْدِي مِنَ الْمَسِيحِ الدَّجَّالِ ‏"‏ ‏.‏ قَالَ قُلْنَا بَلَى ‏.‏ فَقَالَ ‏"‏ الشِّرْكُ الْخَفِيُّ أَنْ يَقُومَ الرَّجُلُ يُصَلِّي فَيُزَيِّنُ صَلاَتَهُ لِمَا يَرَى…

Narrated by Abu Sa‘īd al-Khudri + 1 other version Explained

“The thing that I fear most for my nation is associating others with Allah. I do not say that they will worship the sun or the moon or idols, but deeds done for the sake of anyone other than Allah, and hidden desires.”

"‏ إِنَّ أَخْوَفَ مَا أَتَخَوَّفُ عَلَى أُمَّتِي الإِشْرَاكُ بِاللَّهِ أَمَا إِنِّي لَسْتُ أَقُولُ يَعْبُدُونَ شَمْسًا وَلاَ قَمَرًا وَلاَ وَثَنًا وَلَكِنْ أَعْمَالاً لِغَيْرِ اللَّهِ وَشَهْوَةً خَفِيَّةً ‏"

Narrated by Shaddād ibn Aws