What is the ruling on using the substance (rennet) found in cheeses of unknown origin?

Rennet (Al-Rinit) is a substance extracted from the stomach of a suckling goat kid or lamb. It is added to milk to curdle it and turn it into cheese. The ruling concerning rennet varies depending on its source: if it is taken from an animal slaughtered according to Islamic law (dhakāh shar'iyyah), then it is pure and permissible. However, if it is taken from a dead animal (maytah) or an animal not slaughtered according to Islamic law, there is a difference of opinion among jurists: the majority of Malikis, Shafi'is, and Hanbalis consider it impure, while Abu Hanifa and one narration from Ahmad hold it to be pure. Shaykh al-Islam Ibn Taymiyyah preferred this latter opinion, and accordingly, their cheese is permissible, and the rennet and milk of a dead animal are pure.