A Hadith ascribed to the Prophet Muhammad, Sallallahu alaihi Wasallam, says: "The support of the house is its foundation and the support of the faith [Imaan] is the knowledge of Allah Most High, certitude and a restraining intelligenge." When A'isha, may Allah be pleased with her, asked the Prophet, Sallallahu alaihi Wasallam, : "What is the restraining intelligenge (al-'aql al-qami)"? He answered: "Refraining from disobedience to Allah and eagerness to obey Him."


Culled from al-Qushayri's
"EPISTLE ON SUFISM", in which you can see more on this.