Secret sins Destroy good deeds

روى ابن ماجة (4245) عَنْ ثَوْبَانَ رضي الله عنه عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنَّهُ قَالَ : ( لَأَعْلَمَنَّ أَقْوَامًا مِنْ أُمَّتِي يَأْتُونَ يَوْمَ الْقِيَامَةِ بِحَسَنَاتٍ أَمْثَالِ جِبَالِ تِهَامَةَ بِيضًا ، فَيَجْعَلُهَا اللَّهُ عَزَّ وَجَلَّ هَبَاءً مَنْثُورًا . قَالَ ثَوْبَانُ : يَا رَسُولَ اللَّهِ صِفْهُمْ لَنَا جَلِّهِمْ لَنَا ، أَنْ لَا نَكُونَ مِنْهُمْ وَنَحْنُ لَا نَعْلَمُ . قَالَ : أَمَا إِنَّهُمْ إِخْوَانُكُمْ وَمِنْ جِلْدَتِكُمْ وَيَأْخُذُونَ مِنْ اللَّيْلِ كَمَا تَأْخُذُونَ ، وَلَكِنَّهُمْ أَقْوَامٌ إِذَا خَلَوْا بِمَحَارِمِ اللَّهِ انْتَهَكُوهَا ) .

وهو حيث صحيح ، صححه البوصيري في "مصباح الزجاجة" (4/246) والألباني في "الصحيحة" ، وقال المنذري في الترغيب والترهيب" (3/170) : "رواته ثقات" 

“I certainly know people of my ummah who will come on the Day of Resurrection with good deeds like the mountains of Tihaamah, but Allaah will make them like scattered dust.” Thawbaan said: O Messenger of Allaah, describe them to us and tell us more, so that we will not become of them unknowingly. He said: “They are your brothers and from your race, worshipping at night as you do, but they will be people who, when they are alone, transgress the sacred limits of Allaah.” 

وقد عد ابن حجر الهيتمي رحمه الله في "الزواجر" (3/49) إظْهَار زِيِّ الصَّالِحِينَ فِي الْمَلَأِ ، وَانْتِهَاكُ الْمَحَارِمِ ، وَلَوْ صَغَائِرَ ، فِي الْخَلْوَةِ من الكبائر ، واستشهد بهذا الحديث ، وقال :

" لِأَنَّ مَنْ كَانَ دَأْبُهُ إظْهَارَ الْحَسَنِ وَإِسْرَارَ الْقَبِيحِ ، يَعْظُمُ ضَرَرُهُ وَإِغْوَاؤُهُ لِلْمُسْلِمِينَ ؛ لِانْحِلَالِ رِبْقَةِ التَّقْوَى وَالْخَوْفِ مِنْ عُنُقِهِ " انتهى .

Ibn Hajar al-Haythami (may Allah have mercy on him) in al-Zawaajir (3/49) regarded it as pretending to be righteous outwardly and transgressing the sacred limits; even committing minor sins when one is alone is regarded as something major, and he quoted this hadeeth as evidence, and said: The one whose habit is to make a show of being good and concealing his evil ways  causes more harm and misguidance to the Muslims, because he has no piety and no fear of Allah.  End quote.