Hadith on Martyrs: A man may appear as a martyr, but is not

Umar ibn al-Khattab reported: On the day of Khaybar, some of the companions were saying, “Such person is a martyr. Such person is a martyr.” Then, they passed by a man and they said, “Such person is a martyr.” The Messenger of Allah, peace and blessings be upon him, said, “Surely not! I saw him in the Hellfire because of a cloak or garment he had plundered.” Then, the Prophet said, “O son of al-Khattab, go and announce to people that no one will enter Paradise but a believer.”

Source: Ṣaḥīḥ Muslim 114

Grade: Sahih (authentic) according to Muslim

عُمَرُ بْنُ الْخَطَّابِ قَالَ لَمَّا كَانَ يَوْمُ خَيْبَرَ أَقْبَلَ نَفَرٌ مِنْ صَحَابَةِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالُوا فُلَانٌ شَهِيدٌ فُلَانٌ شَهِيدٌ حَتَّى مَرُّوا عَلَى رَجُلٍ فَقَالُوا فُلَانٌ شَهِيدٌ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَلَّا إِنِّي رَأَيْتُهُ فِي النَّارِ فِي بُرْدَةٍ غَلَّهَا أَوْ عَبَاءَةٍ ثُمَّ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَا ابْنَ الْخَطَّابِ اذْهَبْ فَنَادِ فِي النَّاسِ أَنَّهُ لَا يَدْخُلُ الْجَنَّةَ إِلَّا الْمُؤْمِنُونَ

114 صحيح مسلم كتاب الإيمان باب غلظ تحريم الغلول وأنه لا يدخل الجنة إلا المؤمنون