Hadith: The thing that I fear most for you is minor Shirk (polytheism).' They said: 'What is minor Shirk, O Messenger of Allah?' He said: 'It is Riyā’

عَنْ مَحْمُودِ بْنِ لَبِيدٍ رضي الله عنه أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ:
«إِنَّ أَخْوَفَ مَا أَخَافُ عَلَيْكُمُ الشِّرْكُ الْأَصْغَرُ» قَالُوا: وَمَا الشِّرْكُ الْأَصْغَرُ يَا رَسُولَ اللهِ؟ قَالَ: «الرِّيَاءُ، يَقُولُ اللهُ عز وجل لَهُمْ يَوْمَ الْقِيَامَةِ إِذَا جُزِيَ النَّاسُ بِأَعْمَالِهِمْ: اذْهَبُوا إِلَى الَّذِينَ كُنْتُمْ تُرَاؤُونَ فِي الدُّنْيَا، فَانْظُرُوا هَلْ تَجِدُونَ عِنْدَهُمْ جَزَاءً؟».
[حسن] - [رواه أحمد] - [مسند أحمد: 23630]

Mahmūd ibn Labīd (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"The thing that I fear most for you is minor Shirk (polytheism).' They said: 'What is minor Shirk, O Messenger of Allah?' He said: 'It is Riyā’ (showing off). Allah Almighty will say on the Day of Judgment when He will requite all people for their deeds: "Go to those before whom you used to show off with your deeds in the worldly life and see whether you find any reward with them?"
[Good hadith] - [Narrated by Ahmad] - [Musnad Ahmad - 23630]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informs that what he fears most for his Ummah is minor Shirk, which is showing off, i.e., doing good deeds for the sake of people. Then, he (may Allah's peace and blessings be upon him) informs of the punishment of those who show off with their good deeds as it will be said to them on the Day of Judgment: Go to those for the sake of whom you offered your good deeds and see if they have the ability to reward you for such deeds.