Musnad Ahmad 26:225

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, from Khalid ibn Yazid, from Sa'id ibn Abi Hilal, from Ibrahim ibn 'Ubayd ibn Rifa'ah, that Abu Muhammad informed him—and he was one of the companions of Ibn Mas'ud—that he narrated to him from the Messenger of Allah, may Allah bless him and grant him peace: That the martyrs were mentioned in his presence, so he said: "Indeed, the majority of the martyrs of my nation are those who die upon their beds, and many a person killed between the two ranks, Allah knows best his intention."