مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3749
Musnad of Abdullah bin Mas'ood
It was narrated from Ibraheerm bin 'Ubaid bin Rifa'ah that Abu Muhammad, who was one of the companions of Ibn Mas'ood (رضي الله عنه), told him, narrating from the Messenger of Allah (ﷺ), that Mention of the martyrs was trade in his presence and he said: "Most of the martyrs of my ummah will be people who die in their beds. It may be that the one who dies on the battlefield, Allah knows best what his intention is."
Musnad Ahmad 3772its isnad is da'eef because Ibn Lahee’ah is da'eef]