Musnad Ahmad 30:1744

Ibrahim narrated to us, Ibn al-Mubarak narrated to us, from Wuhayb, 'Umar ibn Muhammad ibn al-Munkadir informed me, from Sumayy, from Abu Salih, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "Whoever dies without having fought [in the way of Allah], and without having spoken to himself about fighting, has died upon a branch of hypocrisy."