Hadithcore
Riyad as-Salihin, 216

'Umar bin Al-Khattab (May Allah bepleased with him) reported:

On the day (of the battle) of Khaibar, some Companions of the Prophet (ﷺ) came and remarked: "So-and-so is a martyr and so-and-so is a martyr". When they came to a man about whom they said: "So-and-so is a martyr," the Prophet (ﷺ) declared, "No. I have seen him in Hell for a mantle (or cloak) which he has stolen". <b>[Muslim]</b>.

وعن عمر بن الخطاب رضي الله عنه قال‏:‏ لما كان يوم خيبر أقبل نفر من أصحاب النبي صلى الله عليه وسلم فقالوا فلان شهيد، وفلان شهيد، حتى مروا على رجل فقالوا ‏:‏ فلان شهيد‏.‏ فقال النبي صلى الله عليه وسلم ‏:‏ كلا إني رأيته في النار في بردة غلها-أو عباءة-” ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏

References1 variant
In-Book Reference
Introduction, Hadith 216
Sharh · explanationclick to expand
<b>Guidance from the Hadith:</b><br> 1) It is impermissible to call a certain person a martyr, because martyrdom in the Sight of Allah Almighty is to fight for the sake of making Allah’s word superior, and what is in the hearts is known to Allah Almighty alone.<br> 2) Being killed in the cause of Allah does not expiate for the violation of others’ rights.