Nay, I saw him in the Fire because of a mantle – or a cloak – that he had taken without right (from the war spoils)

‘Umar ibn al-Khattāb (may Allah be pleased with him) reported that on the Day of the battle of Khaybar, some of the Companions of the Prophet (may Allah's peace and blessings be upon him) came saying: 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 (may Allah's peace and blessings be upon him) remarked: Nay. I saw him in the Fire because of a cloak (from the war spoils) that he had concealed for himself.

Thus, he was punished for it in Hellfire and this great attribute of martyrdom in the cause of Allah was negated from him.