Technically, however, the concept of maslahah mursalah does not apply to the rulings of the Prophet. When there is a Prophetic ruling in favour of a maslahah, it becomes part of the established law, and hence no longer a maslahah mursalah. Historically, the notion of maslahah mursalah originated in the practice of the Companions. This is, of course, not to say that the Prophet did not rule in favour of maslahah, but merely to point out that as a principle of jurisprudence, maslahah mursalah does not apply to the rulings of the Sunnah.
The main support for istislah as a proof and basis of legislation (tashri) comes from Imam Malik, who has given the following reasons in its favour: The Companions have validated it and have formulated the rules of Shari'ah on its basis. When the maslahah is compatible with the objectives of the Lawgiver (maqasid al-shari`) or falls within the genus or category of what the Lawgiver has expressly validated, it must be upheld. For neglecting it under such circumstances is tantamount to neglecting the objectives of the Lawgiver,