[Machine] Allah says: "Indeed, associating others with Allah is a great injustice" and "If you associate partners with Him, your actions will surely become worthless, and you will certainly be among the losers."
قَالَ اللَّهُ تَعَالَى: {إِنَّ الشِّرْكَ لَظُلْمٌ عَظِيمٌ}، {لَئِنْ أَشْرَكْتَ لَيَحْبَطَنَّ عَمَلُكَ وَلَتَكُونَنَّ مِنَ الْخَاسِرِينَ}.
When the Verse: 'It is those who believe and confuse not their belief with wrong (i.e., worshipping others besides Allah): (6.82) was revealed, it became very hard on the companions of the Prophet ﷺ and they said, "Who among us has not confused his belief with wrong (oppression)?" On that, Allah's Apostle said, "This is not meant (by the Verse). Don't you listen to Luqman's statement: 'Verily! Joining others in worship with Allah is a great wrong indeed.' (31.13)
لَمَّا نَزَلَتْ هَذِهِ الآيَةُ {الَّذِينَ آمَنُوا وَلَمْ يَلْبِسُوا إِيمَانَهُمْ بِظُلْمٍ} شَقَّ ذَلِكَ عَلَى أَصْحَابِ النَّبِيِّ ﷺ وَقَالُوا أَيُّنَا لَمْ يَلْبِسْ إِيمَانَهُ بِظُلْمٍ فَقَالَ رَسُولُ اللَّهِ ﷺ إِنَّهُ لَيْسَ بِذَاكَ أَلاَ تَسْمَعُونَ إِلَى قَوْلِ لُقْمَانَ {إِنَّ الشِّرْكَ لَظُلْمٌ عَظِيمٌ}
The Prophet. said, "The biggest of the great sins are: To join others in worship with Allah, to be undutiful to one's parents, and to give a false witness." He repeated it thrice, or said, "....a false statement," and kept on repeating that warning till we wished he would stop saying it. (See Hadith No.7, Vol. 8)
قَالَ النَّبِيُّ ﷺ أَكْبَرُ الْكَبَائِرِ الإِشْرَاكُ بِاللَّهِ وَعُقُوقُ الْوَالِدَيْنِ وَشَهَادَةُ الزُّورِ وَشَهَادَةُ الزُّورِ ثَلاَثًا أَوْ قَوْلُ الزُّورِ فَمَا زَالَ يُكَرِّرُهَا حَتَّى قُلْنَا لَيْتَهُ سَكَتَ
A bedouin came to the Prophet ﷺ and said, "O Messenger of Allah ﷺ! What are the biggest sins?: The Prophet ﷺ said, "To join others in worship with Allah." The bedouin said, "What is next?" The Prophet ﷺ said, "To be undutiful to one's parents." The bedouin said "What is next?" The Prophet ﷺ said "To take an oath 'Al-Ghamus." The bedouin said, "What is an oath 'Al-Ghamus'?" The Prophet ﷺ said, "The false oath through which one deprives a Muslim of his property (unjustly).
جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ ﷺ فَقَالَ يَا رَسُولَ اللَّهِ مَا الْكَبَائِرُ قَالَ الإِشْرَاكُ بِاللَّهِ قَالَ ثُمَّ مَاذَا قَالَ ثُمَّ عُقُوقُ الْوَالِدَيْنِ قَالَ ثُمَّ مَاذَا قَالَ الْيَمِينُ الْغَمُوسُ قُلْتُ وَمَا الْيَمِينُ الْغَمُوسُ قَالَ الَّذِي يَقْتَطِعُ مَالَ امْرِئٍ مُسْلِمٍ هُوَ فِيهَا كَاذِبٌ
A man said, "O Messenger of Allah ﷺ! Shall we be punished for what we did in the Prelslamic Period of ignorance?" The Prophet ﷺ said, "Whoever does good in Islam will not be punished for what he did in the Pre-lslamic Period of ignorance and whoever does evil in Islam will be punished for his former and later (bad deeds).
رَجُلٌ يَا رَسُولَ اللَّهِ أَنُؤَاخَذُ بِمَا عَمِلْنَا فِي الْجَاهِلِيَّةِ قَالَ مَنْ أَحْسَنَ فِي الإِسْلاَمِ لَمْ يُؤَاخَذْ بِمَا عَمِلَ فِي الْجَاهِلِيَّةِ وَمَنْ أَسَاءَ فِي الإِسْلاَمِ أُخِذَ بِالأَوَّلِ وَالآخِرِ