Musannaf Ibn Abi Shayba 23:119

Yazid narrated to us, he said: Al-Awwam ibn Hawshab informed us, from Abu Sadiq, from Ali, who said: "Indeed, Islam has three cornerstones: faith, prayer, and the community (al-jama'ah). No prayer is accepted except with faith, and whoever has faith prays, and whoever prays joins the community. And whoever separates from the community by the span of a hand has indeed removed the yoke of Islam from his neck."