From BiblePortal Wikipedia

Sonna [1]

in Mohammedan law, is, according to the Book of Definitions, the observance of religion in matters respecting which there is no positive and necessary command; also the general practice of the prophets, with some few exceptions. Now this general practice in matters of religion is called the Sonna of guidance, but in those of common occurrence the Sonna of excess. The Sonna of guidance is that by the due performance of which religion is rendered complete, and the dereliction of which is either detestable or sinful. The Sonna of excess is that to embrace which constitutes guidance; that is, it performs, insures good works, but the dereliction of which is neither detestable nor sinful; as, for instance, the custom of the prophet in rising, sitting, putting on his clothes, etc., is not binding, but if followed is meritorious. The Sonna, therefore, comprises the Mohammedan traditions. (See Sunna).