Musnad Ahmad 30:2753

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Mansur, from Abu Hazim, who narrates from Abu Hurayrah—Shu‘bah said: He attributed it to the Prophet (marfu‘) once, then he did not attribute it to him afterwards—that he said: "There is no desertion [of a brother] after three [days]—or beyond three—so whoever deserts [a brother] after three—or beyond three—and dies, he shall enter the Fire."