Musnad Ahmad 30:3445

Yazid narrated to us, Al-Awwam informed us, Abdullah ibn al-Sa’ib narrated to me, from a man of the Ansar, from Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), who said: "The prayer to the prayer that preceded it is an expiation, and the Friday to the Friday that preceded it is an expiation, and the month to the month that preceded it is an expiation, except for three"—he said, and we realized it was a matter that had occurred—"except for associating partners with Allah (shirk), breaking the pledge of allegiance, and abandoning the Sunnah." We said: "O Messenger of Allah, this associating partners with Allah we have known, but what are breaking the pledge of allegiance and abandoning the Sunnah?" He said: "As for breaking the pledge of allegiance, it is that you give a man your pledge, then you fight him with your sword. And as for abandoning the Sunnah, it is departing from the community (al-jama'ah)."