Surah Az-Zumar (The Groups — الزمر) (Ayah 52)

In Verse Az-Zumar 39:47 ^⧉ , it was said: وَبَدَا لَهُم مِّنَ اللَّـهِ مَا لَمْ يَكُونُوا يَحْتَسِبُونَ (And there will appear to them from Allah what they have never imagined.) The famous tabi` i, Sufyan Thawri recited this verse and said, 'Ruined are those who did good deeds only to pretend their piety before people. (He repeated this sentence twice) This verse is about them. They did good deeds in their mortal life to impress people, who took them to be good people. Even they themselves used to deceive themselves by thinking that these deeds will become a source of salvation for them in the Hereafter. But, as these were never performed with unalloyed sincerity ('ikhlas), they deserve no reward in the sight of Allah. So, once they are in the life after death, the punishment would come upon them suddenly against their expectation.' (Qurtubi)

An important instruction concerning 'mushajarat' (disagreements among Sahabah)

Someone asked Rabi' Ibn-ul _-Khaitham about the shahadah (martyrdom) of Sayyidna Husain ؓ . He sighed and recited the Verse Az-Zumar 39:46 ^⧉ قُلِ اللَّـهُمَّ فَاطِرَ‌ السَّمَاوَاتِ وَالْأَرْ‌ضِ عَالِمَ الْغَيْبِ وَالشَّهَادَةِ أَنتَ تَحْكُمُ بَيْنَ عِبَادِكَ ( O Allah, Creator of the heavens and the earth, Knower of the unseen and the seen, You will judge between Your servants...) and said, 'whenever you have a doubt in your heart about the mutual difference of the noble Sahabah, do recite this verse.' Tafsir Ruh-ul-Ma’ ani reports this statement and then says: This teaches us the best etiquette in regard to this issue, and is something one should always bear in mind.