Hadith: 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

عن أبي سعيد الخدري رضي الله عنه مرفوعاً: "ألا أخبركم بما هو أَخْوَفُ عليكم عندي من المسيح الدجال؟ قالوا: بلى يا رسول الله، قال: الشرك الخفي يقوم الرجل فيصلي فَيُزَيِّنُ صلاته لما يرَى من نَظَرِ رَجُلٍ".
[حسن] - [رواه ابن ماجه]

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 improves his prayer because he sees another man watching him."
[Good hadith] - [Narrated by Ibn Majah]

Explanation

The Companions were discussing the trial of the Antichrist, and expressing their fear thereof. The Prophet (may Allah's peace and blessings be upon him) told them that there was a more frightening trial than that of the Antichrist which was hidden polytheism, i.e. being insincere in one's intention and practice, such as improving something that is originally done for Allah's sake so as to attract people's attention.