Musnad Ahmad 30:1971

Husayn narrated to us, Shayban narrated to us, from Mansur, from Abu Hazim, from Abu Hurayrah, who said—and I believe he mentioned it from the Prophet, peace and blessings of Allah be upon him—he said: "There is no boycotting beyond three [days]. Whoever boycotts his brother beyond three [days] and then dies, he shall enter the Fire."